intel: avoid unnecessary front buffer flushing/updating

Before, if we just called glXMakeCurrent() and didn't render anything we'd
still trigger a flushFrontBuffer() call.

Now only set the intel->front_buffer_dirty field at state validation time
just before we draw something.

NOTE: additional calls to intel_check_front_buffer_rendering() might be
needed if I missed some rendering paths.

src/mesa/drivers/dri/i915/intel_tris.c

@@ -1088,6 +1088,7 @@ intelRenderStart(GLcontext * ctx)
 {
    struct intel_context *intel = intel_context(ctx);
 
+   intel_check_front_buffer_rendering(intel);
    intel->vtbl.render_start(intel_context(ctx));
    intel->vtbl.emit_state(intel);
 }

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

@@ -34,6 +34,7 @@
 #include "brw_context.h"
 #include "brw_state.h"
 #include "intel_batchbuffer.h"
+#include "intel_buffers.h"
 
 /* This is used to initialize brw->state.atoms[].  We could use this
  * list directly except for a single atom, brw_constant_buffer, which
@@ -324,6 +325,8 @@ void brw_validate_state( struct brw_context *brw )
       }
    }
 
+   intel_check_front_buffer_rendering(intel);
+
    /* Make sure that the textures which are referenced by the current
     * brw fragment program are actually present/valid.
     * If this fails, we can experience GPU lock-ups.

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

@@ -132,6 +132,25 @@ intel_get_cliprects(struct intel_context *intel,
 }
 
 
+/**
+ * Check if we're about to draw into the front color buffer.
+ * If so, set the intel->front_buffer_dirty field to true.
+ */
+void
+intel_check_front_buffer_rendering(struct intel_context *intel)
+{
+   const struct gl_framebuffer *fb = intel->ctx.DrawBuffer;
+   if (fb->Name == 0) {
+      /* drawing to window system buffer */
+      if (fb->_NumColorDrawBuffers > 0) {
+         if (fb->_ColorDrawBufferIndexes[0] == BUFFER_FRONT_LEFT) {
+	    intel->front_buffer_dirty = GL_TRUE;
+	 }
+      }
+   }
+}
+
+
 /**
  * Update the hardware state for drawing into a window or framebuffer object.
  *
@@ -202,8 +221,6 @@ intel_draw_buffer(GLcontext * ctx, struct gl_framebuffer *fb)
 	       intel_batchbuffer_flush(intel->batch);
 	    intel->front_cliprects = GL_TRUE;
 	    colorRegions[0] = intel_get_rb_region(fb, BUFFER_FRONT_LEFT);
-
-	    intel->front_buffer_dirty = GL_TRUE;
 	 }
 	 else {
 	    if (!intel->constant_cliprect && intel->front_cliprects)

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

@@ -45,6 +45,8 @@ extern struct intel_region *intel_readbuf_region(struct intel_context *intel);
 
 extern struct intel_region *intel_drawbuf_region(struct intel_context *intel);
 
+extern void intel_check_front_buffer_rendering(struct intel_context *intel);
+
 extern void intel_draw_buffer(GLcontext * ctx, struct gl_framebuffer *fb);
 
 extern void intelInitBufferFuncs(struct dd_function_table *functions);

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

@@ -501,6 +501,8 @@ intel_map_unmap_framebuffer(struct intel_context *intel,
       else
          intel_renderbuffer_unmap(intel, fb->_StencilBuffer->Wrapped);
    }
+
+   intel_check_front_buffer_rendering(intel);
 }
 
 /**