Added assert for opengles thread safety (flutter/engine#56585)

This assert would have blocked the following bug:
flutter#158535

This shouldn't be landed until
flutter/engine#56573 lands.

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Author: gaaclarke

engine/src/flutter/impeller/renderer/backend/gles/proc_table_gles.cc

@@ -140,6 +140,10 @@ ProcTableGLES::ProcTableGLES(  // NOLINT(google-readability-function-size)
 
   capabilities_ = std::make_shared<CapabilitiesGLES>(*this);
 
+  // This this will force glUseProgram to only be used on one thread in debug
+  // builds to identify threading violations in the engine.
+  UseProgram.enforce_one_thread = true;
+
   is_valid_ = true;
 }
 

engine/src/flutter/impeller/renderer/backend/gles/proc_table_gles.h

@@ -6,7 +6,9 @@
 #define FLUTTER_IMPELLER_RENDERER_BACKEND_GLES_PROC_TABLE_GLES_H_
 
 #include <functional>
+#include <mutex>
 #include <string>
+#include <thread>
 
 #include "flutter/fml/logging.h"
 #include "flutter/fml/mapping.h"
@@ -99,6 +101,14 @@ struct GLProc {
   ///
   bool log_calls = false;
 
+  //----------------------------------------------------------------------------
+  /// Whether the OpenGL call asserts it is only used from / one thread in
+  /// IMPELLER_DEBUG builds.
+  ///
+  /// This is used to block drawing calls from happening anywhere but the raster
+  /// thread.
+  bool enforce_one_thread = false;
+
   //----------------------------------------------------------------------------
   /// @brief      Call the GL function with the appropriate parameters. Lookup
   ///             the documentation for the GL function being called to
@@ -118,6 +128,16 @@ struct GLProc {
       FML_LOG(IMPORTANT) << name
                          << BuildGLArguments(std::forward<Args>(args)...);
     }
+    if (enforce_one_thread) {
+      static std::thread::id allowed_thread;
+      static std::once_flag flag;
+      std::call_once(flag,
+                     []() { allowed_thread = std::this_thread::get_id(); });
+      FML_CHECK(std::this_thread::get_id() == allowed_thread)
+          << "This symbol is expected to be called from one thread, the raster "
+             "thread. As of this addition, the design of the engine should be "
+             "using non-raster threads only for uploading images.";
+    }
 #endif  // defined(IMPELLER_DEBUG) && !defined(NDEBUG)
     return function(std::forward<Args>(args)...);
   }