glmemory: Use glBlitFramebuffer() instead of glCopyTexImage2D() if available

glmemory: Use glBlitFramebuffer() instead of glCopyTexImage2D() if available

glBlitFramebuffer() is new GLES3/GL3 API. They are probably often
implemented in terms of each other, and glBlitFramebuffer() can
potentially be implemented more flexible. Let's use it if available.

Also it actually works on imx6 with DirectVIV textures.

Based on a patch by Haihua Hu <jared.hu@nxp.com> from
https://github.com/Freescale/meta-freescale/blob/master/recipes-multimedia/gstreamer/gstreamer1.0-plugins-bad/

https://bugzilla.gnome.org/show_bug.cgi?id=779070