[cogl] framebuffer: Support the GL_RED texture workaround when querying bits

[Neil Roberts <nroberts src gnome org>]

commit 425cfb2675912a2cbcaaaeed7c2196d563948222
Author: Neil Roberts <neil linux intel com>
Date:   Thu Dec 13 15:55:05 2012 +0000

    framebuffer: Support the GL_RED texture workaround when querying bits
    
    When a component-alpha texture is made using a GL3 context a GL_RED
    texture is actually used and a swizzle is set up to hide it. However
    if a framebuffer is then bound to that texture then when the bits are
    queried this workaround will leak out of the API. To fix this it now
    detects the situation and reports the number of red bits as the number
    of alpha bits.
    
    Reviewed-by: Robert Bragg <robert linux intel com>

 cogl/driver/gl/cogl-framebuffer-gl.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)
---
diff --git a/cogl/driver/gl/cogl-framebuffer-gl.c b/cogl/driver/gl/cogl-framebuffer-gl.c
index 2be8b96..d8a015d 100644
--- a/cogl/driver/gl/cogl-framebuffer-gl.c
+++ b/cogl/driver/gl/cogl-framebuffer-gl.c
@@ -948,6 +948,16 @@ _cogl_framebuffer_init_bits (CoglFramebuffer *framebuffer)
       GE( ctx, glGetIntegerv (GL_STENCIL_BITS, &framebuffer->bits.stencil) );
     }
 
+  /* If we don't have alpha textures then the alpha bits are actually
+   * stored in the red component */
+  if ((ctx->private_feature_flags &
+       COGL_PRIVATE_FEATURE_ALPHA_TEXTURES) == 0 &&
+      framebuffer->type == COGL_FRAMEBUFFER_TYPE_OFFSCREEN &&
+      framebuffer->format == COGL_PIXEL_FORMAT_A_8)
+    {
+      framebuffer->bits.alpha = framebuffer->bits.red;
+      framebuffer->bits.red = 0;
+    }
 
   COGL_NOTE (OFFSCREEN,
              "RGBA/D/S Bits for framebuffer[%p, %s]: %d, %d, %d, %d, %d, %d",