Rework overlay driver interface.

Takes multiple images, each with individual positions, textures, etc.
Start adding loading of alternative descs.
Add alpha_mod support.

Change alpha per object if pressed.
Merge pull request #350 from lioncash/master

[Android] Update the build instruction file for Android
Merge pull request #351 from lioncash/master

[Android] Fix the markdown syntax for the numeric list in the build instructions.
Add range_mod.
Fixup overlay interface for D3D9.
Add example overlay.
Fix overlay calculations with rect.
Merge pull request #352 from libretro/overlay-rework

Overlay rework
[Android] Revise core location in Building in Eclipse.md