Disable vertex attrib arrays in overlay_gl (fixes #1298)

This is a fix for the OpenGL overlay which disables and enables the vertex attrib arrays inside drawContext.
The problem it fixes is that some games leave some of the arrays enabled between frames, which I think (not an OpenGL expert) causes the input to the vertex shader to essentially be arbitrary.
Some times it is manifested as extreme stretching of the overlay texture other times the texture is not visible at all. The texture itself is rendering properly in all cases but in the non-working games it doesn't get rendered properly into the back buffer.

I've been testing the current master and the fix on different native games on Ubuntu 14.04 64 bits.
The following games work (the overlay shows up at the main menu) both before and after the fix:
Braid, Brutal Legend, Dota2, Eets Munchies, FTL, Narcissu 1st & 2nd, Superbrothers Sword & Sworcery, Trine 2.
The following games work only with the fix:
Estranged Act I, Half-Life 2, Limbo, No More Room in Hell, Portal, Psychonauts, Super Meat Boy, Synergy, Team Fortress 2.
Mark of the Ninja is not working with either but I haven't looked further into it.

Probably should use GLboolean instead of unsigned char to make flag character obvious.