Frontend: Declare unconditional support and lack of support of certain features.

* Hi-res support and reverse stereo are unconditionally not supported, and thus never compiled in.
* 16-bit sound and SNES clip windows are unconditionally supported, and thus always compiled in.
tile.cpp: Grab the depths of layers to be written outside of tile drawing loops.

GFX being a global variable, conditional accesses to GFX.Z1 and GFX.Z2 inside the loops cannot be automatically put out of the loop by the compiler.
tile.cpp: Perform computations to draw 2 pixels at once.

CANNOT BE SENT UPSTREAM, due to the code needed to support SNES hi-res mode not being there.

On MIPS, memory loads for multiple pixels may be launched concurrently because there are enough registers to hold all of them. It is advantageous to load more because, if a register's value is needed immediately after it is loaded from memory and it is not loaded yet, the program waits, doing nothing.