Support images with premultiplied alpha

[Alloyed]

This is a crosspost of ocornut/imgui#1035

TL;DR: I want to use imgui with premultiplied alpha images, and there's no way (that I can find) to tell love-imgui to do that.

[slages]

I made a change to automatically apply the premultiplied alpha blend mode when imgui.Image or imgui.ImageButton is used to draw a Canvas, it should fix your issue!

I didn't made a new release, so you'll need to build a new version from source.

[Alloyed]

Yep, it works! This is maybe less flexible than I imagined (I couldn't have a normal Image use premultiplied alpha for example) but I wasn't planning on doing that sort of thing anyways.