cogl-framebuffer: Add some missing GL defines

Since 755cce33a7 the framebuffer code is using the GL enums
GL_DEPTH_ATTACHMENT and GL_DEPTH_COMPONENT16. These aren't available
directly under GLES except with the OES suffix so we need to define
them manually as we do with the other framebuffer constants.

clutter/cogl/cogl/cogl-framebuffer.c

@@ -74,6 +74,12 @@
 #ifndef GL_DEPTH_STENCIL
 #define GL_DEPTH_STENCIL        0x84F9
 #endif
+#ifndef GL_DEPTH_ATTACHMENT
+#define GL_DEPTH_ATTACHMENT     0x8D00
+#endif
+#ifndef GL_DEPTH_COMPONENT16
+#define GL_DEPTH_COMPONENT16    0x81A5
+#endif
 
 typedef enum {
   _TRY_DEPTH_STENCIL = 1L<<0,