OpenGL: Make use of persistent buffer maps in buffer cache downloads

Persistent buffer maps were already used by the texture cache, this extends their usage for the buffer cache.

In my testing, using the memory maps for uploads was slower than the existing "ImmediateUpload" path, so the memory map usage is limited to downloads for the time being.

src/video_core/renderer_vulkan/vk_buffer_cache.h

@@ -157,6 +157,7 @@ struct BufferCacheParams {
     static constexpr bool USE_MEMORY_MAPS = true;
     static constexpr bool SEPARATE_IMAGE_BUFFER_BINDINGS = false;
     static constexpr bool IMPLEMENTS_ASYNC_DOWNLOADS = true;
+    static constexpr bool USE_MEMORY_MAPS_FOR_UPLOADS = true;
 };
 
 using BufferCache = VideoCommon::BufferCache<BufferCacheParams>;