intel: Fallback to software if drawable size is > MaxRenderbufferSize

This prevents the mystery blank window if, for example, glxgears is
resized larger than 2048 wide on 915.  Since the Intel drivers in Mesa
7.6 lack GTT mapped fallbacks, the performance is a slideshow at
best.  On Mesa 7.7 and later the performance is much better.

src/mesa/drivers/dri/intel/intel_buffers.c

@@ -172,10 +172,15 @@ intel_draw_buffer(GLcontext * ctx, struct gl_framebuffer *fb)
       return;
    }
 
-   /*
-    * How many color buffers are we drawing into?
+   /* How many color buffers are we drawing into?
+    *
+    * If there are zero buffers or the buffer is too big, don't configure any
+    * regions for hardware drawing.  We'll fallback to software below.  Not
+    * having regions set makes some of the software fallback paths faster.
     */
-   if (fb->_NumColorDrawBuffers == 0) {
+   if ((fb->Width > ctx->Const.MaxRenderbufferSize)
+       || (fb->Height > ctx->Const.MaxRenderbufferSize)
+       || (fb->_NumColorDrawBuffers == 0)) {
       /* writing to 0  */
       colorRegions[0] = NULL;
       intel->constant_cliprect = GL_TRUE;