Before this change, the pixel object buffer is unbound just after
getting a pointer by glMapBuffer. This seemed suspicious.
This change fixes to do all pixel manipulations once between
glMapBuffer and glUnmapBuffer without changing a bound buffer.
This might fix a wrong rendering on some machines, but I am not
sure.
Updates #993
This change is an experimental implementation to use Pixel Buffer
Objects. This reduces calls of glTexSubImage2D.
This works only on desktops. Unfortunately WebGL does not have
this features. Mobiles can have PBO as of OpenGL ES 3.
Updates #976
