This is a measure to avoid crashes on command list reading as an empty
command_list is considered a NOP.

Nothing in this header relies on common_funcs or the memory manager.

This gets rid of reliance on indirect inclusions in the OpenGL caches.

# Conflicts:
#	src/video_core/engines/kepler_memory.cpp
#	src/video_core/engines/maxwell_3d.cpp
#	src/video_core/morton.cpp
#	src/video_core/morton.h
#	src/video_core/renderer_opengl/gl_global_cache.cpp
#	src/video_core/renderer_opengl/gl_global_cache.h
#	src/video_core/renderer_opengl/gl_rasterizer_cache.cpp

Instead of holding a reference that will get invalidated by
dma_pushbuffer.pop(), hold it as a copy. This doesn't have any
performance cost since CommandListHeader is 8 bytes long.

As fetching command list headers and and the list of command headers is a fixed 1:1 relation now, they can be implemented within a single call.
This cleans up the Step() logic quite a bit.

Fetching every u32 from memory leads to a big overhead. So let's fetch all of them as a block if possible.
This reduces the Memory::* calls by the dma_pusher by a factor of 10.

- More accurate impl., fixes Undertale (among other games).