Make blending (in scaling) respect alpha

[abewieland]

When sampling from multiple pixels to implement scaling (only relevant for anti-aliasing), the sampled values should be multiplied by their alpha value to prevent colors "bleeding" from almost-transparent regions. Consider this image, which consists of a square in the center with color #00ff00ff and a border of #ff000001:

Without multiplying by the alpha, we can get unexpected red in the border region, as seen here with anti-aliasing on (so bicubic):

This is because the alpha values are higher there from the green square, but the red component is stronger than it should be.

With the PR gives:

[artemsen]

I didn't even think that someone would fix this part of the code.
Thanks!