more fixes for mesadisplay

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -4336,7 +4336,7 @@ set_draw_buffer(const RenderBuffer &rb) {
       break;
 
     default:
-      GLP(DrawBuffer)(GL_FRONT_AND_BACK);
+      break;
     }
   }
 

panda/src/mesadisplay/osMesaGraphicsBuffer.cxx

@@ -38,6 +38,7 @@ OsMesaGraphicsBuffer(GraphicsPipe *pipe,
   GraphicsBuffer(pipe, name, properties, x_size, y_size, flags, gsg, host)
 {
   _type = GL_UNSIGNED_BYTE;
+  _draw_buffer_type = RenderBuffer::T_front;
 }
 
 ////////////////////////////////////////////////////////////////////