SDL_GPU Shader and Instructions

Added source sharder files for the SDL_GPU Backend and instructions on how to compile them

backends/sdlgpu3/build_instructions.txt

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+1) Compile the raw shader files to SPIRV:
+
+    glslc -o vertex.spv -c shader.vert
+    glslc -o fragment.spv -c shader.frag
+
+
+2) Build SDL_shadercross (https://github.com/libsdl-org/SDL_shadercross)
+
+
+3-A) Compiling for the Vulkan Driver:
+
+    Nothing to do, you just need the previous vertex.spv/fragment.spv, proceed to step 4
+
+
+3-B) Compiling for the DirectX 12 Driver:
+
+    ./shadercross vertex.spv -s SPIRV -d DXBC -t vertex -e main -o vertex.dxbc
+    ./shadercross fragment.spv -s SPIRV -d DXBC -t fragment -e main -o fragment.dxbc
+
+    Proceed to step 4
+
+
+3-C) Compiling for Metal (On windows you'll need the Metal Developer Tools for Windows, on linux you might use wine, but I never tested it):
+
+    ./shadercross vertex.spv -s SPIRV -d MSL -t vertex -e main -o vertex.metal
+    ./shadercross fragment.spv -s SPIRV -d MSL -t fragment -e main -o fragment.metal
+
+    xcrun -sdk macosx metal -o vertex.ir -c vertex.metal
+    xcrun -sdk macosx metal -o fragment.ir -c fragment.metal
+    xcrun -sdk macosx metallib -o vertex.metallib -c vertex.ir
+    xcrun -sdk macosx metallib -o fragment.metallib -c fragment.ir
+
+    Proceed to step 4
+
+
+4) Either find a way to load the shader bytecode from file, or use a tool like https://notisrac.github.io/FileToCArray/ to convert the file to a uint8_t array

backends/sdlgpu3/shader.frag

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+#version 450 core
+layout(location = 0) out vec4 fColor;
+
+layout(set=2, binding=0) uniform sampler2D sTexture;
+
+layout(location = 0) in struct {
+    vec4 Color;
+    vec2 UV;
+} In;
+
+void main()
+{
+    fColor = In.Color * texture(sTexture, In.UV.st);
+}

backends/sdlgpu3/shader.vert

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+#version 450 core
+layout(location = 0) in vec2 aPos;
+layout(location = 1) in vec2 aUV;
+layout(location = 2) in vec4 aColor;
+
+layout(set=1,binding=0) uniform UBO {
+    vec2 uScale;
+    vec2 uTranslate;
+} ubo;
+
+layout(location = 0) out struct {
+    vec4 Color;
+    vec2 UV;
+} Out;
+
+void main()
+{
+    Out.Color = aColor;
+    Out.UV = aUV;
+    gl_Position = vec4(aPos * ubo.uScale + ubo.uTranslate, 0, 1);
+    gl_Position.y *= -1.0f;
+}