tile.cpp: Re-macro some code that had been expanded in DrawTile16Add.
tile.cpp: Use MIPS assembly for WRITE_4PIXELS16[_FLIPPED], part of DrawTile16.
tile.cpp: Inline WRITE_4PIXELS16[_FLIPPED] into its caller.
tile.cpp: Do not check whether a block of 4 pixels contains an opaque one.

This is counter-intuitive, but in tiles that are not fully transparent, it is more likely that a pixel is opaque. The change in this commit also eliminates one immediately-needed memory load per 4 pixels.
Remove the HeadMask and TailMask global variables.

Computing the necessary values in TILE_CLIP_PREAMBLE cuts down on the cycles needed versus a load from memory that is unlikely to be cached at the time of every access.