Alpha blended draws with `canvas=true` causes alpha to be double-multiplied

[michaelgoeke]

I plugged in push, and I'm drawing text and some other graphics elements that use alpha values.
The draw call in applyShaders uses the default blend mode with seems to be multiplied the alpha twice (e.g. if it was 50%, 0.5, it would now render with 25%, 0.25 instead)

I believe the fix is to set love.graphics.setBlendMode("alpha", "premultiplied") before the draw, and restore it afterwards.