Fix blur with alpha looking wonky

[ErikReider]

We probably would need to interpolate between the original texture and the blurred texture when adjusting the alpha of the blur?

Image of alpha 0.9, blur passes 3, and blur radius set to 5:

[WillPower3309]

Hmm this worked fine on swayfx with my animations branch (where we update the blur alpha with the tex alpha for fade in/ out). Wonder what's going wrong here