Add hint to specify Vulkan API version to use

[TheSniperFan]

Currently SDL_gpu_vulkan.c hardcodes API version 1.0 when creating the Vulkan instance for the program. This creates a problem for my use-case.

I'd like to use Slang for shader development, but SPIR-V 1.0 support is experimental. I tried two solutions and both work:

1. Slang's experimental compiler flags which make Slang transpile to GLSL first and then to SPIR-V, rather than compiling directly to SPIR-V.
2. Add a hint to SDL to allow creating instances with higher API versions.

I looked at the Vulkan documentation (specifically this and this). It says "Deprecated items will still work in current Vulkan implementations", which makes me prefer option 2. A Vulkan application requesting 1.3, but using only 1.0 features should work just fine. And using a hint doesn't change SDL's default behavior.

Since I don't know why this was hardcoded, I figured I'd first ask if there's any chance of this merge request going anywhere. Before I waste time cleaning something up that's not going to be merged for reasons I didn't know of.

[thatcosmonaut]

We hardcode to 1.0 to provide maximum driver compatibility. I'm totally fine with allowing an opt-in to higher versions.

[slouken]

I think it's reasonable to allow specifying the full version and will be more intuitive for users. This is easily implemented with, e.g.

if (SDL_sscanf(hint, "%d.%d", &major, &minor) == 2) {
    ...
} else {
    // Parse error...
}

[TheSniperFan]

This needs a bit more time in the oven, I think.
Setting the API version during instance creation does allow Slang-compiled shaders to be loaded without the validation layer complaining. However, Slang might emit SPIR-V that relies on physical device features that would have to be specified during device creation.
I'll see if there's a sane, non-intrusive way to implement that tomorrow.

[slouken]

The string should be delimited by . so it looks like a natural version number.