intel: Avoid dri2 GetBuffers round-trips for internal Viewport calls.

This gets us the savings for driver-internal viewport calls that
dd1c68f151 was attempting, without relying
on Xlib internals or clients handling X events.

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

@@ -307,9 +307,11 @@ intel_viewport(GLcontext *ctx, GLint x, GLint y, GLsizei w, GLsizei h)
     if (!driContext->driScreenPriv->dri2.enabled)
 	return;
 
-    intel_update_renderbuffers(driContext, driContext->driDrawablePriv);
-    if (driContext->driDrawablePriv != driContext->driReadablePriv)
-	intel_update_renderbuffers(driContext, driContext->driReadablePriv);
+    if (!intel->internal_viewport_call) {
+       intel_update_renderbuffers(driContext, driContext->driDrawablePriv);
+       if (driContext->driDrawablePriv != driContext->driReadablePriv)
+	  intel_update_renderbuffers(driContext, driContext->driReadablePriv);
+    }
 
     old_viewport = ctx->Driver.Viewport;
     ctx->Driver.Viewport = NULL;

src/mesa/drivers/dri/intel/intel_context.h

@@ -180,6 +180,7 @@ struct intel_context
    struct intel_region *front_region;
    struct intel_region *back_region;
    struct intel_region *depth_region;
+   GLboolean internal_viewport_call;
 
    /**
     * This value indicates that the kernel memory manager is being used

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

@@ -184,7 +184,9 @@ intel_meta_set_passthrough_transform(struct intel_context *intel)
    intel->meta.saved_vp_height = ctx->Viewport.Height;
    intel->meta.saved_matrix_mode = ctx->Transform.MatrixMode;
 
+   intel->internal_viewport_call = GL_TRUE;
    _mesa_Viewport(0, 0, ctx->DrawBuffer->Width, ctx->DrawBuffer->Height);
+   intel->internal_viewport_call = GL_FALSE;
 
    _mesa_MatrixMode(GL_PROJECTION);
    _mesa_PushMatrix();
@@ -206,8 +208,10 @@ intel_meta_restore_transform(struct intel_context *intel)
 
    _mesa_MatrixMode(intel->meta.saved_matrix_mode);
 
+   intel->internal_viewport_call = GL_TRUE;
    _mesa_Viewport(intel->meta.saved_vp_x, intel->meta.saved_vp_y,
 		  intel->meta.saved_vp_width, intel->meta.saved_vp_height);
+   intel->internal_viewport_call = GL_FALSE;
 }
 
 /**