Support for external allocators: (KhronosGroup#748)

* The newly introduced API surface area matches that of VMA's advanced
   usage and another hand-rolled memory allocator in a content-heavy application.
* All suballocator callbacks -- allocate, bind image, bind buffer, map, unmap and
  free -- are expected to guard VkDeviceMemory operations within a mutex.
* Each texture now also keeps track of its VkDeviceMemory offset.
* The 64 bit allocationId is to be used as a book-keeping measure by external
   suballocator callbacks to keep track of and free up suballocations. The external
   allocator can use a hashtable (ala std::unordered_map in C++) to keep track of
   the page(s) alloted to this suballocation. ('Pages' here refers to potential sparse
   bindings).

* Add a VkLoadTest for suballocation callbacks

include/ktxvulkan.h

@@ -114,8 +114,9 @@ typedef struct ktxVulkanTexture
     VkImageLayout imageLayout; /*!< Layout of the created image. Has the same
                                     value as @p layout parameter passed to the
                                     loader. */
-    VkDeviceMemory deviceMemory; /*!< The memory allocated for the image on
-                                  the Vulkan device. */
+    VkDeviceMemory deviceMemory; /*!< The memory (sub)allocation for the
+                                  image on the Vulkan device. Will not be
+                                  used with suballocators.*/
     VkImageViewType viewType; /*!< ViewType corresponding to @p image. Reflects
                                    the dimensionality, cubeness and arrayness
                                    of the image. */
@@ -124,15 +125,41 @@ typedef struct ktxVulkanTexture
     uint32_t depth; /*!< The depth of the image. */
     uint32_t levelCount; /*!< The number of MIP levels in the image. */
     uint32_t layerCount; /*!< The number of array layers in the image. */
+    uint64_t allocationId; /*!< An id referencing suballocation(s). */
 } ktxVulkanTexture;
 
+typedef uint64_t(*ktxVulkanTexture_subAllocatorAllocMemFuncPtr)(VkMemoryAllocateInfo* allocInfo, VkMemoryRequirements* memReq, uint64_t* pageCount);
+typedef VkResult(*ktxVulkanTexture_subAllocatorBindBufferFuncPtr)(VkBuffer buffer, uint64_t allocId);
+typedef VkResult(*ktxVulkanTexture_subAllocatorBindImageFuncPtr)(VkImage image, uint64_t allocId);
+typedef VkResult(*ktxVulkanTexture_subAllocatorMemoryMapFuncPtr)(uint64_t allocId, uint64_t pageNumber, VkDeviceSize *mapLength, void** dataPtr);
+typedef void (*ktxVulkanTexture_subAllocatorMemoryUnmapFuncPtr)(uint64_t allocId, uint64_t pageNumber);
+typedef void (*ktxVulkanTexture_subAllocatorFreeMemFuncPtr)(uint64_t allocId);
+/**
+ * @class ktxVulkanTexture_subAllocatorCallbacks
+ * @~English
+ * @brief Struct that contains all callbacks necessary for suballocation.
+ *
+ * These pointers must all be provided for upload or destroy to occur using suballocator callbacks.
+ */
+typedef struct {
+    ktxVulkanTexture_subAllocatorAllocMemFuncPtr allocMemFuncPtr; /*!< Pointer to the memory procurement function. Can suballocate one or more pages. */
+    ktxVulkanTexture_subAllocatorBindBufferFuncPtr bindBufferFuncPtr; /*!< Pointer to bind-buffer-to-suballocation(s) function. */
+    ktxVulkanTexture_subAllocatorBindImageFuncPtr bindImageFuncPtr; /*!< Pointer to bind-image-to-suballocation(s) function. */
+    ktxVulkanTexture_subAllocatorMemoryMapFuncPtr memoryMapFuncPtr; /*!< Pointer to function for mapping the memory of a specific page. */
+    ktxVulkanTexture_subAllocatorMemoryUnmapFuncPtr memoryUnmapFuncPtr; /*!< Pointer to function for unmapping the memory of a specific page. */
+    ktxVulkanTexture_subAllocatorFreeMemFuncPtr freeMemFuncPtr; /*!< Pointer to the free procurement function. */
+} ktxVulkanTexture_subAllocatorCallbacks;
+
+KTX_API ktx_error_code_e KTX_APIENTRY
+ktxVulkanTexture_Destruct_WithSuballocator(ktxVulkanTexture* This, VkDevice device,
+                                           const VkAllocationCallbacks* pAllocator,
+                                           ktxVulkanTexture_subAllocatorCallbacks* subAllocatorCallbacks);
+
 KTX_API void KTX_APIENTRY
 ktxVulkanTexture_Destruct(ktxVulkanTexture* This, VkDevice device,
                           const VkAllocationCallbacks* pAllocator);
 
 
-
-
 /**
  * @class ktxVulkanDeviceInfo
  * @~English
@@ -208,6 +235,13 @@ ktxVulkanDeviceInfo_Destruct(ktxVulkanDeviceInfo* This);
 KTX_API void KTX_APIENTRY
 ktxVulkanDeviceInfo_Destroy(ktxVulkanDeviceInfo* This);
 KTX_API KTX_error_code KTX_APIENTRY
+ktxTexture_VkUploadEx_WithSuballocator(ktxTexture* This, ktxVulkanDeviceInfo* vdi,
+                                       ktxVulkanTexture* vkTexture,
+                                       VkImageTiling tiling,
+                                       VkImageUsageFlags usageFlags,
+                                       VkImageLayout finalLayout,
+                                       ktxVulkanTexture_subAllocatorCallbacks* subAllocatorCallbacks);
+KTX_API KTX_error_code KTX_APIENTRY
 ktxTexture_VkUploadEx(ktxTexture* This, ktxVulkanDeviceInfo* vdi,
                       ktxVulkanTexture* vkTexture,
                       VkImageTiling tiling,