gl_texture_cache: Workaround slow PBO downloads on radeonsi

There's an optimization bug on non-git mesa versions where not
specifying GL_CLIENT_STORAGE_BIT causes very slow reads on the CPU
side.

Add this bit for all vendors.

src/video_core/renderer_opengl/gl_texture_cache.h

@@ -122,7 +122,7 @@ private:
     bool has_broken_texture_view_formats = false;
 
     StagingBuffers upload_buffers{GL_MAP_WRITE_BIT, GL_MAP_WRITE_BIT | GL_MAP_FLUSH_EXPLICIT_BIT};
-    StagingBuffers download_buffers{GL_MAP_READ_BIT, GL_MAP_READ_BIT};
+    StagingBuffers download_buffers{GL_MAP_READ_BIT | GL_CLIENT_STORAGE_BIT, GL_MAP_READ_BIT};
 
     OGLTexture null_image_1d_array;
     OGLTexture null_image_cube_array;