softgpu: Attempt to implement GE_PROJMAP_UV.

Looks okay, not sure if it's fully correct.
Clip in PPGe, fixing issues in softgpu.

We were just drawing wrong, so softgpu was correct.
softgpu: Use the display, not current, framebuffer.

Fixes blinking in save dialogs and probably other bugs.
headless: default to single core.

Was zero before, causing divide by zero issues.
softgpu: Don't mask out so much of fb stride.

Seems like anything from 4 is supported in 8888, most likely it just needs
to align to 16 bytes.  Values above 1024 work, but e.g. 2044 seems buggy.

Fixes the map on Hexyz Force (rendered at 80 stride.)
softgpu: Properly swizzle 16 bit textures.

Fixes for example the graphics in Fieldrunners.
softgpu: Retain accuracy in depth for tests.

Fixes intro/menu issues in Phantasy Star Portable 1/2, and text in the
first, both of which now look correct.
softgpu: Fix black objects with bilinear filtering.

Dividing by 256 gives us a range of 0-253, and incorrectly gets zero for
some components (guessing alpha, primarily?) in some games.

Dividing by 255 has a speed hit, so a slight bias to the top left seems
like a decent performance compromise (and it's a very slight bias.)
Allow auto frameskip to skip > 1 frame.
Merge pull request #4849 from unknownbrackets/softgpu

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