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)

Logically, the GSP functionality required to access these features from applications are now sufficiently emulated, too (a lot of guesswork went into that, however).

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.