Support memsets of framebuffers as uploads.

Technically should clear stencil too... maybe it'd be better to handle
separately.
Support swizzled framebuffer downloads.

Used in God Eater 2 when showing the load save screen.
Use a separate func for gpu memset().
Don't savestate replacement funcs.

This makes it so we can reorder them as we like, which is nicer, and also
makes sure there won't be weird compatibility problems or etc.
Merge pull request #6148 from unknownbrackets/gpu-blocktransfer

Support memset and swizzling memcpy on framebuffers