[cogl/cogl-1.20] cogl-framebuffer-gl: Work again on GLESv2

[Jasper St. Pierre <jstpierre src gnome org>]

commit f8cce5f6cb2958a4074f87cd345bfe46e0dda6e1
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Mon Apr 20 12:08:29 2015 -0700

    cogl-framebuffer-gl: Work again on GLESv2

 cogl/driver/gl/cogl-framebuffer-gl.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/cogl/driver/gl/cogl-framebuffer-gl.c b/cogl/driver/gl/cogl-framebuffer-gl.c
index 793b10b..609cfb3 100644
--- a/cogl/driver/gl/cogl-framebuffer-gl.c
+++ b/cogl/driver/gl/cogl-framebuffer-gl.c
@@ -107,6 +107,12 @@
 #ifndef GL_PACK_INVERT_MESA
 #define GL_PACK_INVERT_MESA 0x8758
 #endif
+#ifndef GL_BACK_LEFT
+#define GL_BACK_LEFT                           0x0402
+#endif
+#ifndef GL_BACK_RIGHT
+#define GL_BACK_RIGHT                          0x0403
+#endif
 
 #ifndef GL_COLOR
 #define GL_COLOR 0x1800
@@ -245,6 +251,9 @@ _cogl_framebuffer_gl_flush_stereo_mode_state (CoglFramebuffer *framebuffer)
   if (framebuffer->type == COGL_FRAMEBUFFER_TYPE_OFFSCREEN)
     return;
 
+  if (!ctx->glDrawBuffer)
+    return;
+
   /* The one-shot default draw buffer setting in _cogl_framebuffer_gl_bind
    * must have already happened. If not it would override what we set here. */
   g_assert (ctx->was_bound_to_onscreen);