renderer-native-gles3: Add cpp error if gl.h was included

On some architectures, including both GLES3/gl3.h GL/gl.h will cause
compilation issues due to incompatible type definitions. To avoid
running into that issue while building on other architectures, make
sure we haven't included GL/gl.h by accident.

https://bugzilla.gnome.org/show_bug.cgi?id=788695

src/backends/native/meta-renderer-native-gles3.c

@@ -34,6 +34,14 @@
 #include "backends/meta-gles3.h"
 #include "backends/meta-gles3-table.h"
 
+/*
+ * GL/gl.h being included may conflit with gl3.h on some architectures.
+ * Make sure that hasn't happened on any architecture.
+ */
+#ifdef GL_VERSION_1_1
+#error "Somehow included OpenGL headers when we shouldn't have"
+#endif
+
 static EGLImageKHR
 create_egl_image (MetaEgl       *egl,
                   EGLDisplay     egl_display,