i965: Always intel_prepare_render() after invalidating front buffers.

Fixes glean/texture_srgb, which hit recursive-flush prevention
assertions in vbo_exec_FlushVertices.

This probably hurts the performance of front buffer rendering, but
very few people in their right mind do front buffer rendering.

Fixes Glean's texture_srgb test.

Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Acked-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

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

@@ -65,6 +65,7 @@ intelDrawBuffer(struct gl_context * ctx, GLenum mode)
        * (including the fake front) before we start rendering again.
        */
       dri2InvalidateDrawable(brw->driContext->driDrawablePriv);
+      intel_prepare_render(brw);
    }
 }
 
@@ -80,6 +81,7 @@ intelReadBuffer(struct gl_context * ctx, GLenum mode)
        * (including the fake front) before we start reading again.
        */
       dri2InvalidateDrawable(brw->driContext->driReadablePriv);
+      intel_prepare_render(brw);
    }
 }