Opcode decoding: handle missing opcodes 0x88 etc.

Hardware testing shows that they do the same thing as the 0x80 family of
opcodes: they draw quads.
Video backends: warn on usage of GL_DRAW_QUADS_2.

It's not normally used, so if it shows up, it could indicate a CPU emulation
bug.
Merge pull request #355 from magumagu/gx-missing-opcode

Opcode decoding: handle missing opcodes 0x88 etc.
MMIO: Lazy-initialize read/write handlers since most are not actually used