Set a default shader entry point

The default should be the entrypoint generated by SDL_shadercross. That way it doesn't need to be hand-specified in the common workflow.

(cherry picked from commit dcb97a5f49a336f871828672be7a905502778dbc)

src/gpu/d3d12/SDL_gpu_d3d12.c

@@ -8103,7 +8103,6 @@ static void D3D12_INTERNAL_InitBlitResources(
     shaderCreateInfo.code_size = sizeof(D3D12_FullscreenVert);
     shaderCreateInfo.stage = SDL_GPU_SHADERSTAGE_VERTEX;
     shaderCreateInfo.format = SDL_GPU_SHADERFORMAT_DXBC;
-    shaderCreateInfo.entrypoint = "main";
 
     renderer->blitVertexShader = D3D12_CreateShader(
         (SDL_GPURenderer *)renderer,

src/gpu/metal/SDL_gpu_metal.m

@@ -840,6 +840,10 @@ static MetalLibraryFunction METAL_INTERNAL_CompileShader(
     dispatch_data_t data;
     id<MTLFunction> function;
 
+    if (!entrypoint) {
+        entrypoint = "main0";
+    }
+
     if (format == SDL_GPU_SHADERFORMAT_MSL) {
         NSString *codeString = [[NSString alloc]
             initWithBytes:code

src/gpu/vulkan/SDL_gpu_vulkan.c

@@ -606,7 +606,7 @@ typedef struct VulkanSampler
 typedef struct VulkanShader
 {
     VkShaderModule shaderModule;
-    const char *entrypointName;
+    char *entrypointName;
     SDL_GPUShaderStage stage;
     Uint32 numSamplers;
     Uint32 numStorageTextures;
@@ -3175,7 +3175,7 @@ static void VULKAN_INTERNAL_DestroyShader(
         vulkanShader->shaderModule,
         NULL);
 
-    SDL_free((void *)vulkanShader->entrypointName);
+    SDL_free(vulkanShader->entrypointName);
     SDL_free(vulkanShader);
 }
 
@@ -6694,7 +6694,6 @@ static SDL_GPUShader *VULKAN_CreateShader(
     VkResult vulkanResult;
     VkShaderModuleCreateInfo vkShaderModuleCreateInfo;
     VulkanRenderer *renderer = (VulkanRenderer *)driverData;
-    size_t entryPointNameLength;
 
     vulkanShader = SDL_malloc(sizeof(VulkanShader));
     vkShaderModuleCreateInfo.sType = VK_STRUCTURE_TYPE_SHADER_MODULE_CREATE_INFO;
@@ -6714,10 +6713,11 @@ static SDL_GPUShader *VULKAN_CreateShader(
         CHECK_VULKAN_ERROR_AND_RETURN(vulkanResult, vkCreateShaderModule, NULL);
     }
 
-    entryPointNameLength = SDL_strlen(createinfo->entrypoint) + 1;
-    vulkanShader->entrypointName = SDL_malloc(entryPointNameLength);
-    SDL_utf8strlcpy((char *)vulkanShader->entrypointName, createinfo->entrypoint, entryPointNameLength);
-
+    const char *entrypoint = createinfo->entrypoint;
+    if (!entrypoint) {
+        entrypoint = "main";
+    }
+    vulkanShader->entrypointName = SDL_strdup(entrypoint);
     vulkanShader->stage = createinfo->stage;
     vulkanShader->numSamplers = createinfo->num_samplers;
     vulkanShader->numStorageTextures = createinfo->num_storage_textures;