i965: Add an env var for forcing window system MSAA.

Sometimes it would be nice to benchmark some app with MSAA versus not, but
it doesn't offer the controls you want.  Just provide a handy knob to
force the issue.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/intel_screen.c

@@ -975,6 +975,11 @@ intelCreateBuffer(__DRIscreen * driScrnPriv,
 
    _mesa_initialize_window_framebuffer(fb, mesaVis);
 
+   if (screen->winsys_msaa_samples_override != -1) {
+      num_samples = screen->winsys_msaa_samples_override;
+      fb->Visual.samples = num_samples;
+   }
+
    if (mesaVis->redBits == 5)
       rgbFormat = MESA_FORMAT_B5G6R5_UNORM;
    else if (mesaVis->sRGBCapable)
@@ -1335,6 +1340,16 @@ __DRIconfig **intelInitScreen2(__DRIscreen *psp)
 
    intelScreen->hw_has_swizzling = intel_detect_swizzling(intelScreen);
 
+   const char *force_msaa = getenv("INTEL_FORCE_MSAA");
+   if (force_msaa) {
+      intelScreen->winsys_msaa_samples_override =
+         intel_quantize_num_samples(intelScreen, atoi(force_msaa));
+      printf("Forcing winsys sample count to %d\n",
+             intelScreen->winsys_msaa_samples_override);
+   } else {
+      intelScreen->winsys_msaa_samples_override = -1;
+   }
+
    set_max_gl_versions(intelScreen);
 
    /* Notification of GPU resets requires hardware contexts and a kernel new

src/mesa/drivers/dri/i965/intel_screen.h

@@ -62,6 +62,8 @@ struct intel_screen
     */
    unsigned program_id;
 
+   int winsys_msaa_samples_override;
+
    struct {
       struct ra_regs *regs;