Do not alter pixels outside mask when drawing text on an image with transparency

[radarhere]

Resolves #5431

The issue reports that drawing text onto an RGBA image, and then removing the transparency by converting that image to L, a rectangle around the text has been affected by the draw operation. Testing, this was not the case before #4566.

Why is this the case?

When drawing text onto an image, a mask is used.

#4566 changed how the mask was blended into an image so that if the image was transparent, it's RGB values were not considered - so that black transparent pixels do not combine with white opaque text to give gray pixels. Comparing the output of the test from that PR,

____	Without PR 4566	With PR 4566
RGBA

However, it turns out that this was applied too broadly, ignoring the RGB values the whole of the mask area - if the above results are converted to L,

____	Without PR 4566	With PR 4566
L

It should only be applied to the parts where the mask is not zero.

____	With This PR
RGBA
L