Bug 1528394 - Reduce includes of GLContext.h. r=lsalzman

Splitting part of OGLShaderProgram.h out into OGLShaderConfig.h makes it
easier to keep GLContext.h included in OGLShaderProgram.h for inlining
purposes.

Differential Revision: https://phabricator.services.mozilla.com/D20017

gfx/layers/opengl/OGLShaderProgram.cpp

@@ -5,6 +5,7 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "OGLShaderProgram.h"
+
 #include <stdint.h>  // for uint32_t
 #include <sstream>   // for ostringstream
 #include "gfxEnv.h"