opengles emulator: replace the readback at end of frame with blit

That replaces the readback to host we do at the end of each frame
with two blits on the GPU, one to copy the pixels into a texture
and another to render the texture to another in order to flip
the image.

Change-Id: I7e0e10493d38944d0b613e245023f34236d3dfc4

tools/emulator/opengl/host/libs/libOpenglRender/WindowSurface.h

@@ -45,6 +45,7 @@ private:
     WindowSurface();
 
     void copyToColorBuffer();  // copy pbuffer content with readback+download
+    void blitToColorBuffer();  // copy pbuffer content with texload and blit
     bool resizePbuffer(unsigned int p_width, unsigned int p_height);
 
 private: