GPU: `colorAttachmentDescriptions` should be `const`

The `colorAttachmentDescriptions` field of `SDL_GPUGraphicsPipelineAttachmentInfo` should be marked `const`,
similarly to the `vertexBindings` and `vertexAttributes` fields of `SDL_GPUVertexInputState`.

include/SDL3/SDL_gpu.h

@@ -1158,7 +1158,7 @@ typedef struct SDL_GPUColorAttachmentDescription
 
 typedef struct SDL_GPUGraphicsPipelineAttachmentInfo
 {
-    SDL_GPUColorAttachmentDescription *colorAttachmentDescriptions;
+    const SDL_GPUColorAttachmentDescription *colorAttachmentDescriptions;
     Uint32 colorAttachmentCount;
     SDL_bool hasDepthStencilAttachment;
     SDL_GPUTextureFormat depthStencilFormat;

src/gpu/d3d11/SDL_gpu_d3d11.c

@@ -1250,7 +1250,7 @@ static void D3D11_ReleaseGraphicsPipeline(
 static ID3D11BlendState *D3D11_INTERNAL_FetchBlendState(
     D3D11Renderer *renderer,
     Uint32 numColorAttachments,
-    SDL_GPUColorAttachmentDescription *colorAttachments)
+    const SDL_GPUColorAttachmentDescription *colorAttachments)
 {
     ID3D11BlendState *result;
     D3D11_BLEND_DESC blendDesc;

src/gpu/metal/SDL_gpu_metal.m

@@ -997,7 +997,7 @@ static SDL_GPUGraphicsPipeline *METAL_CreateGraphicsPipeline(
         MetalShader *vertexShader = (MetalShader *)pipelineCreateInfo->vertexShader;
         MetalShader *fragmentShader = (MetalShader *)pipelineCreateInfo->fragmentShader;
         MTLRenderPipelineDescriptor *pipelineDescriptor;
-        SDL_GPUColorAttachmentBlendState *blendState;
+        const SDL_GPUColorAttachmentBlendState *blendState;
         MTLVertexDescriptor *vertexDescriptor;
         Uint32 binding;
         MTLDepthStencilDescriptor *depthStencilDescriptor;