Incorrect alpha treatment when RenderToImageBegin...End over transparent background

[eugeneloza2factor]

When using RenderToImageBegin...RenderToImageEnd and the image is transparent, alpha is treated incorrectly.

Expected result:

Current result:

Minimal reproduction (all code in Start):

offscreen-render-alpha.zip

BlackTransparentBackground := TRGBAlphaImage.Create(900, 300);
BlackTransparentBackground.Clear(Vector4(0, 0, 0, 0)); // using Vector4(0, 0, 0, 1) fixes the bug, but disallows transparent background
WhiteBox := TRGBAlphaImage.Create(200, 200);
WhiteBox.Clear(Vector4(1, 1, 1, 1));

BackgroundDrawable := TDrawableImage.Create(BlackTransparentBackground, true, true);
WhiteBoxDrawable := TDrawableImage.Create(WhiteBox, true, true);

BackgroundDrawable.RenderToImageBegin;
WhiteBoxDrawable.Color := Vector4(1, 1, 1, 0.9);
WhiteBoxDrawable.Draw(50, 50);
WhiteBoxDrawable.Color := Vector4(1, 1, 1, 0.6);
WhiteBoxDrawable.Draw(250, 50);
WhiteBoxDrawable.Color := Vector4(1, 1, 1, 0.4);
WhiteBoxDrawable.Draw(450, 50);
WhiteBoxDrawable.Color := Vector4(1, 1, 1, 0.1);
WhiteBoxDrawable.Draw(650, 50);
BackgroundDrawable.RenderToImageEnd;