mesa: fix glPushAttrib(0) / glPopAttrib() error

If the glPushAttrib() mask value was zero we didn't actually push
anything onto the attribute stack.  A subsequent glPopAttrib() call
would generate a GL_STACK_UNDERFLOW error.  Now push a dummy attribute
in that case to prevent the error.

Mesa now matches nvidia's behavior.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

src/mesa/main/attrib.c

@@ -177,6 +177,10 @@ struct texture_state
 };
 
 
+/** An unused GL_*_BIT value */
+#define DUMMY_BIT 0x10000000
+
+
 /**
  * Allocate new attribute node of given type/kind.  Attach payload data.
  * Insert it into the linked list named by 'head'.
@@ -253,6 +257,15 @@ _mesa_PushAttrib(GLbitfield mask)
    /* groups specified by the mask. */
    head = NULL;
 
+   if (mask == 0) {
+      /* if mask is zero we still need to push something so that we
+       * don't get a GL_STACK_UNDERFLOW error in glPopAttrib().
+       */
+      GLuint dummy = 0;
+      if (!push_attrib(ctx, &head, DUMMY_BIT, sizeof(dummy), &dummy))
+         goto end;
+   }
+
    if (mask & GL_ACCUM_BUFFER_BIT) {
       if (!push_attrib(ctx, &head, GL_ACCUM_BUFFER_BIT,
                        sizeof(struct gl_accum_attrib),
@@ -928,6 +941,10 @@ _mesa_PopAttrib(void)
       }
 
       switch (attr->kind) {
+         case DUMMY_BIT:
+            /* do nothing */
+            break;
+
          case GL_ACCUM_BUFFER_BIT:
             {
                const struct gl_accum_attrib *accum;