[mutter] cogl: correct check for COGL_OFFSCREEN_DISABLE_DEPTH_AND_STENCIL

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit d992722c24cbbeaedfcf97bb186689208de91869
Author: Thomas Hindoe Paaboel Andersen <phomes gmail com>
Date:   Fri Nov 15 21:34:12 2019 +0100

    cogl: correct check for COGL_OFFSCREEN_DISABLE_DEPTH_AND_STENCIL
    
    In _cogl_offscreen_gl_allocate we only want to perform certain actions if
    COGL_OFFSCREEN_DISABLE_DEPTH_AND_STENCIL is not set in create_flags.
    
    We perfrom this check with:
    
    if (!offscreen->create_flags & COGL_OFFSCREEN_DISABLE_DEPTH_AND_STENCIL)
    
    which is not correct as we negate the create_flags before the bitwise &.
    
    It happens to work as intended though, as CoglOffscreenFlags only has one
    element, and that element has the value 1. If the flag is not set then the
    nagation of create_flags is true and the bitwise and with the element value
    is true as well.
    If any flag is set then the negation will give 0 and the bitwise & will be
    false.
    
    So while it works correctly it is fragile as either additional flags or a
    change in the enum element value will break this check. This patch makes
    things a bit more safe by adding parentheses to let the bitwise & happen
    before the negation.
    
    Definition of the enum:
    
    typedef enum
    {
      COGL_OFFSCREEN_DISABLE_DEPTH_AND_STENCIL = 1
    } CoglOffscreenFlags;
    
    https://gitlab.gnome.org/GNOME/mutter/merge_requests/938

 cogl/cogl/driver/gl/cogl-framebuffer-gl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/cogl/cogl/driver/gl/cogl-framebuffer-gl.c b/cogl/cogl/driver/gl/cogl-framebuffer-gl.c
index 343793999..b956a8314 100644
--- a/cogl/cogl/driver/gl/cogl-framebuffer-gl.c
+++ b/cogl/cogl/driver/gl/cogl-framebuffer-gl.c
@@ -865,7 +865,7 @@ _cogl_offscreen_gl_allocate (CoglOffscreen *offscreen,
     {
       fb->samples_per_pixel = gl_framebuffer->samples_per_pixel;
 
-      if (!offscreen->create_flags & COGL_OFFSCREEN_DISABLE_DEPTH_AND_STENCIL)
+      if (!(offscreen->create_flags & COGL_OFFSCREEN_DISABLE_DEPTH_AND_STENCIL))
         {
           /* Record that the last set of flags succeeded so that we can
              try that set first next time */