Bug 1656813 - Set correct blend mode for non-premultiplied alpha. r=gw

I believe this is correct for blending a non-premuliplied source into a premultiplied destination. Only the source color part should be different to normal blending, since it needs to be multiplied by the source alpha channel.

Differential Revision: https://phabricator.services.mozilla.com/D85716

gfx/wr/webrender/src/device/gl.rs

@@ -3532,7 +3532,7 @@ impl Device {
     pub fn set_blend_mode_alpha(&mut self) {
         self.set_blend_factors(
             (gl::SRC_ALPHA, gl::ONE_MINUS_SRC_ALPHA),
-            (gl::ONE, gl::ONE),
+            (gl::ONE, gl::ONE_MINUS_SRC_ALPHA),
         );
     }