GPU framebuffer emulation improvements

Documents and implements a lot of the GPU registers properly, including:
* framebuffer configuration
* memory fills (used for framebuffer clearing)
* display transfers (used for displaying the rendered scene to screen)

Along with these changes, this PR fixes a number of issues the old code had, including:
* Improper choice of default framebuffer parameters (the current choice doesn't really emulate correct behavior either, but it's not worse in any regard but works for a wider range of applications)
* Incorrect color component order for the bottom screen
* Illogical and limited implementation of the GSP ReadHWRegs service.

This PR depends on #30.