GLES2: Fix (re-implement) multi-texture in OpenGLES2 codepath.

With this implementation, mipmapping must be disabled in the OpenGLES2
codepath to avoid black textures.

Texture mirror flag is enabled in the in OpenGLES2 codepath to fix
messed up textures like the horse's cheeks in both Zelda games.

extracted from
https://github.com/mupen64plus-ae/mupen64plus-ae/commit/33d250b926141dfd1de72d19b61c8e1b3d7ffda8
GLES2: Use config file to disable mipmapping rather than hard code.
GLES2: Fix (re-implement) fog/alpha shaders in OpenGLES2 codepath.

extracted from
https://github.com/mupen64plus-ae/mupen64plus-ae/commit/33d250b926141dfd1de72d19b61c8e1b3d7ffda8
Remove unneeded macro in osal_opengl.
Merge pull request #1 from littleguy77/gles2-shader-texture-fixes

Gles2 shader texture fixes