Red mushroom textures are inconsistent with vanilla

[Beatso]

Description of bug

In the vanilla red mushroom block texture, the white dots are each in a lighter "plus" shape:

In the variated textures, the background is kept exactly the same and just the dots move around, meaning the dots don't have the lighter area around them:

Solution

Have a lighter area around the variated textures, like the vanilla texture.

Relates to

#25