vulkan gpu: set the sample count for depth prepass

Fixes https://github.com/libsdl-org/SDL/issues/14500

src/gpu/vulkan/SDL_gpu_vulkan.c

@@ -7189,6 +7189,8 @@ static VkRenderPass VULKAN_INTERNAL_FetchRenderPass(
     key.sampleCount = VK_SAMPLE_COUNT_1_BIT;
     if (numColorTargets > 0) {
         key.sampleCount = SDLToVK_SampleCount[((VulkanTextureContainer *)colorTargetInfos[0].texture)->header.info.sample_count];
+    } else if (numColorTargets == 0 && depthStencilTargetInfo != NULL) {
+        key.sampleCount = SDLToVK_SampleCount[((VulkanTextureContainer *)depthStencilTargetInfo->texture)->header.info.sample_count];
     }
 
     key.numColorTargets = numColorTargets;