mesa: fix assertion in _mesa_drawbuffers()

Fixes failed assertion when _mesa_update_draw_buffers() was called
with GL_DRAW_BUFFER == GL_FRONT_AND_BACK.  The piglit gl30basic hit
this.

Cc: "10.2" <[email protected]>

Reviewed-by: Ian Romanick <[email protected]>

1 files changed, 3 insertions, 2 deletions

diff --git a/src/mesa/main/buffers.c b/src/mesa/main/buffers.c
index b13a7af65a6..6b4fac9f58d 100644
--- a/src/mesa/main/buffers.c
+++ b/src/mesa/main/buffers.c
@@ -494,10 +494,11 @@ _mesa_drawbuffers(struct gl_context *ctx, GLuint n, const GLenum *buffers,
    }
 
    /*
-    * If n==1, destMask[0] may have up to four bits set.
+    * destMask[0] may have up to four bits set
+    * (ex: glDrawBuffer(GL_FRONT_AND_BACK)).
     * Otherwise, destMask[x] can only have one bit set.
     */
-   if (n == 1) {
+   if (_mesa_bitcount(destMask[0]) > 1) {
       GLuint count = 0, destMask0 = destMask[0];
       while (destMask0) {
          GLint bufIndex = ffs(destMask0) - 1;