Add dummy GL texture flag & direct object access

 - Add dummy GL texture support to allow libobs texture references to be
   created for GL without

 - Add a texture_getobj function to allow the retrieval of the
   context-specific object, such as the D3D texture pointer, or the
   OpenGL texture object handle.

 - Also cleaned up the export stuff.  I realized it was all totally
   superfluous.  Kind of a dumb moment, but nice to clean it up
   regardless.
Minor fix to GL texture cleanup

Don't clear any data if it's a dummy texture.
Fix CMakeLists.txt for each project (my fault)
Initial Implementation of glx/xcomposite based capture
fixup! Initial Implementation of glx/xcomposite based capture
fixup! Initial Implementation of glx/xcomposite based capture
fixup! Initial Implementation of glx/xcomposite based capture
fixup! Initial Implementation of glx/xcomposite based capture
fixup! Initial Implementation of glx/xcomposite based capture