Fix a bunch of things in softgpu (save dialogs, swizzle, bilinear, depth)

The commits each have notes, with the end result:
 * Depth testing seems correct now in Phantasy Star dialogs at least.  Still something funny in Tales of Eternia.  I'm not sure if it should always be v2.z, or at least in throughmode?  But at least when they match we should retain accuracy, which is common (>= test was failing.)
 * Save dialogs now show properly without glitches (it was actually incorrectly working in gles partially.)
 * 16-bit textures are now swizzled correctly.  Probably 32 also?
 * Render-to-texture works properly in more places (stride was being masked incorrectly, verified on a PSP.)
 * Bilinear filtering was 255^2 / 256^2, which lost bits and caused black in Tales of Eternia's world map, so made it (1 + 255)^2 / 256^2 to retain speed.
 * Attempted to implement GE_PROJMAP_UV.  Least confident about this one, but I think it works?
 * Displays the *display* buffer, not the current render target.  Fixes flickering in save dialogs and transitions in some games.

-[Unknown]