Bug 697757 - WebGL context fix for alpha being improperly set on OSX. r=jgilbert

If we request alpha and it isn't enabled, or we request it off and it gets
enabled, we now fail the pbuffer creation and fall back to FBOs. This is bad for
performance but allows us to pass the conformance test in this case.

content/canvas/test/webgl/failing_tests_mac.txt

@@ -1,4 +1,3 @@
-conformance/context/context-attributes-alpha-depth-stencil-antialias.html
 conformance/context/premultiplyalpha-test.html
 conformance/glsl/misc/glsl-function-nodes.html
 conformance/glsl/misc/glsl-long-variable-names.html
@@ -6,7 +5,6 @@ conformance/glsl/misc/shader-with-256-character-identifier.frag.html
 conformance/glsl/misc/shader-with-long-line.html
 conformance/programs/program-test.html
 conformance/state/gl-object-get-calls.html
-conformance/textures/tex-input-validation.html
 conformance/textures/texture-mips.html
 conformance/textures/texture-npot.html
 conformance/more/conformance/quickCheckAPI-S_V.html

gfx/thebes/GLContextProviderCGL.mm

@@ -497,6 +497,20 @@ CreateOffscreenPBufferContext(const gfxIntSize& aSize,
         return nsnull;
     }
 
+    // If we ask for any of these to be on/off and we get the opposite, we stop
+    // creating a pbuffer and instead create an FBO.
+    GLint alphaBits, depthBits, stencilBits;
+    [pbFormat getValues: &alphaBits forAttribute: NSOpenGLPFAAlphaSize forVirtualScreen: 0];
+    [pbFormat getValues: &depthBits forAttribute: NSOpenGLPFADepthSize forVirtualScreen: 0];
+    [pbFormat getValues: &stencilBits forAttribute: NSOpenGLPFAStencilSize forVirtualScreen: 0];
+    if ((alphaBits && !aFormat.alpha) || (!alphaBits && aFormat.alpha) ||
+        (depthBits && !aFormat.alpha) || (!depthBits && aFormat.depth) ||
+        (stencilBits && !aFormat.stencil) || (!stencilBits && aFormat.stencil)) 
+    {
+        [pbFormat release];
+        return nsnull;
+    }
+
     NSOpenGLPixelBuffer *pb = [[NSOpenGLPixelBuffer alloc]
                                initWithTextureTarget:LOCAL_GL_TEXTURE_2D
                                textureInternalFormat:(aFormat.alpha ? LOCAL_GL_RGBA : LOCAL_GL_RGB)