OpenGL2: Add OpenGL ES 2.0+ support

This mainly targets OpenGL ES 2.0 but it also supports compiling GLSL as
ESSL 3.00. It's missing support for framebuffer objects which should be
possible on ES 2. (Though using renderbuffers instead of textures.)

opengl1 cvars that are not supported will display a message and disable
the cvar. This has not been reviewed for new opengl2 cvars. Enabling
cvars may cause rendering issues. Some of the broken cvars may be
possible to support using OpenGL ES 3 features.

The game displays okay with the default cvars.

code/renderergl2/tr_postprocess.c

@@ -290,6 +290,8 @@ static qboolean RB_UpdateSunFlareVis(void)
 		ri.Printf(PRINT_DEVELOPER, "Waited %d iterations\n", iter);
 	}
 	
+	// Note: On desktop OpenGL this is a sample count (glRefConfig.occlusionQueryTarget == GL_SAMPLES_PASSED)
+	// but on OpenGL ES this is a boolean (glRefConfig.occlusionQueryTarget == GL_ANY_SAMPLES_PASSED)
 	qglGetQueryObjectuiv(tr.sunFlareQuery[tr.sunFlareQueryIndex], GL_QUERY_RESULT, &sampleCount);
 	return sampleCount > 0;
 }