radeon: port more front fixes from intel.

Port fixes to read buffer from front.

src/mesa/drivers/dri/radeon/radeon_common.c

@@ -823,6 +823,17 @@ void radeonDrawBuffer( GLcontext *ctx, GLenum mode )
 
 void radeonReadBuffer( GLcontext *ctx, GLenum mode )
 {
+	if ((ctx->DrawBuffer != NULL) && (ctx->DrawBuffer->Name == 0)) {
+		struct radeon_context *const rmesa = RADEON_CONTEXT(ctx);
+		const GLboolean was_front_buffer_reading = rmesa->is_front_buffer_reading;
+		rmesa->is_front_buffer_reading = (mode == GL_FRONT_LEFT)
+					|| (mode == GL_FRONT);
+
+		if (!was_front_buffer_reading && rmesa->is_front_buffer_reading) {
+			radeon_update_renderbuffers(rmesa->dri.context,
+						    rmesa->dri.context->driReadablePriv);
+	 	}
+	}
 	/* nothing, until we implement h/w glRead/CopyPixels or CopyTexImage */
 	if (ctx->ReadBuffer == ctx->DrawBuffer) {
 		/* This will update FBO completeness status.

src/mesa/drivers/dri/radeon/radeon_common_context.c

@@ -487,7 +487,10 @@ radeon_update_renderbuffers(__DRIcontext *context, __DRIdrawable *drawable)
 		struct radeon_renderbuffer *stencil_rb;
 
 		i = 0;
-		if (draw->color_rb[0]) {
+		if ((radeon->is_front_buffer_rendering ||
+		     radeon->is_front_buffer_reading ||
+		     !draw->color_rb[1])
+		    && draw->color_rb[0]) {
 			attachments[i++] = __DRI_BUFFER_FRONT_LEFT;
 			attachments[i++] = radeon_bits_per_pixel(draw->color_rb[0]);
 		}

src/mesa/drivers/dri/radeon/radeon_common_context.h

@@ -467,6 +467,15 @@ struct radeon_context {
     */
    GLboolean is_front_buffer_rendering;
 
+   /**
+    * Track whether front-buffer is the current read target.
+    *
+    * This is closely associated with is_front_buffer_rendering, but may
+    * be set separately.  The DRI2 fake front buffer must be referenced
+    * either way.
+    */
+   GLboolean is_front_buffer_reading;
+
    /* info for radeon_clear_tris() */
    struct {
       struct gl_array_object *arrayObj;