Don't bother writing an empty symbol map.

For example, when accidentally opening an invalid file.
softgpu: Don't redraw every pixel of a line 16x.

Fixes the insane slowness when processing lines.  Now it's just regular
super slow.
softgpu: Avoid a divide by 0 in line interpolation.

Fixes crash in Persona 2 loading screens.
softgpu: Fix lines drawn upward or leftward.
Merge pull request #5158 from unknownbrackets/softgpu

Fix a bunch of issues in line drawing in softgpu