vulkan: Don't set the opaque bit on transparent windows

If window transparency was requested via the SDL_WINDOW_TRANSPARENT flag, don't set the opaque bit on the swapchain composite alpha value. Fixes transparent windows when using the Vulkan renderer (e.g. testsprite --transparent).

src/render/vulkan/SDL_render_vulkan.c

@@ -2150,7 +2150,7 @@ static VkResult VULKAN_CreateSwapChain(SDL_Renderer *renderer, int w, int h)
     swapchainCreateInfo.imageUsage = VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT | VK_IMAGE_USAGE_TRANSFER_DST_BIT;
     swapchainCreateInfo.imageSharingMode = VK_SHARING_MODE_EXCLUSIVE;
     swapchainCreateInfo.preTransform = rendererData->surfaceCapabilities.currentTransform;
-    swapchainCreateInfo.compositeAlpha = VK_COMPOSITE_ALPHA_OPAQUE_BIT_KHR;
+    swapchainCreateInfo.compositeAlpha = (renderer->window->flags & SDL_WINDOW_TRANSPARENT) ? 0 : VK_COMPOSITE_ALPHA_OPAQUE_BIT_KHR;
     swapchainCreateInfo.presentMode = presentMode;
     swapchainCreateInfo.clipped = VK_TRUE;
     swapchainCreateInfo.oldSwapchain = rendererData->swapchain;