Points cannot be selected atm in editmode, this patch just shows the
selection from sculptmode in editmode.

Since the selection in sculptmode is a float, a point is considered
selected as soon as the float selection is above 0.0f.

Implementation: this piggy-back on the existing drawing via
overlay_edit_curve_point.glsl which requires a "data" VBO which holds
flags for selection (next to others such as "active" - which we also
have to take care of later).