Handle MULTIPLY with alpha in WebGL

[davepagurek]

Resolves #5854

Changes:

• Uses the MULTIPLY blend mode settings that Pixi.js uses
• Updates an existing blend test that ignored alpha (only fully opaque blue multiplied with fully opaque red would yield black; unfortunately, this is how 2D mode works, so this is different than before)
• Updates tests that check if WebGL blending matches 2D to test with some transparency

Note that 2D and WebGL MULTIPLY modes work differently now! The 2D mode has a bunch of quirks where it blends semi transparent colors with white and makes them opaque before multiplication. This PR chooses to handle MULTIPLY in WebGL in a more expected way. If we decide we want to preserve the existing alpha-ignoring method for everything but text, I can update this code to only use the new blend function when drawing text.

Screenshots of the change:

Before	After

PR Checklist

• npm run lint passes
• [Inline documentation] is included / updated
• [Unit tests] are included / updated