Properly implement GPU screenshot for GLES.

Have to use GL_RGBA/GL_UNSIGNED_BYTE as well as reading from back buffer
only.
Make C++ compiler happy.
Fix libretro GL core screenshots.
Clarify SET_MESSAGE.
Add RETRO_ENVIRONMENT_GET_SAVE_DIRECTORY
Fix a typo on the previous commit
Merge pull request #537 from fr500/master

Add  RETRO_ENVIRONMENT_GET_SAVE_DIRECTORY
Update libretro.h docs. Fix conditional string check.
Simplify parsing of --size.
Don't reset volume settings on driver reinit.
Allow toggling fullscreen from bind in RGUI.
D3D9: use new during init so that constructors are run
Merge pull request #542 from OV2/master

D3D9: use new during init so that constructors are run