dx9: Compensate for viewport w/h adjustments.

Fixes text in Final Fantasy 4, world map in Star Ocean, etc.
dx9: Fix a crash on textures used as RT.

We don't always have a texture pointer.
dx9: Correct framebuffer download scaling.

Fixes the menu background in Tales of Phantasia X.
Merge pull request #7196 from unknownbrackets/gpu-minor

dx9: Compensate for viewport w/h adjustments
Merge https://github.com/hrydgard/ppsspp