beginAlphaMaskedCaster

Switches the depth pass to the alpha-tested caster program for a cutout-textured sub-draw: binds texture on unit 0 and loads cutoff. The caller binds its own texcoord attribute (location 1) and calls endAlphaMaskedCaster afterwards. Returns false - leaving the opaque program active - when the variant is unavailable or the texture fails to bind (the sub-draw then casts a solid silhouette).