video_core, core: Move pixel download to its own thread

This uses the mailbox model to move pixel downloading to its own thread, eliminating Nvidia's warnings and (possibly) making use of GPU copy engine.

To achieve this, we created a new mailbox type that is different from the presentation mailbox in that it never discards a rendered frame.

Also, I tweaked the projection matrix thing so that it can just draw the frame upside down instead of having the CPU flip it.

src/video_core/CMakeLists.txt

@@ -23,6 +23,8 @@ add_library(video_core STATIC
     regs_texturing.h
     renderer_base.cpp
     renderer_base.h
+    renderer_opengl/frame_dumper_opengl.cpp
+    renderer_opengl/frame_dumper_opengl.h
     renderer_opengl/gl_rasterizer.cpp
     renderer_opengl/gl_rasterizer.h
     renderer_opengl/gl_rasterizer_cache.cpp