Change equation of alpha blending

[bvssvni]

Currently, the equation for alpha blending is (taken from Gfx):

new_dest_alpha = src_alpha + dest_alpha

Glium uses new_dest_alpha = src_alpha * src_alpha + dest_alpha * (1 - src_alpha).

According to https://en.wikipedia.org/wiki/Alpha_compositing, both equations are incorrect. The correct equation for alpha is:

new_dest_alpha = src_alpha + dest_alpha * (1 - src_alpha)