Change header file macro prefix

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add u2gl_ prefix to common OpenGL calls

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Add glenz comments to README

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>