Add an API to render images with transparency

[nibirubingus]

Currently, the textures module and widgets.makeButtonLabelText only allow for non-transparent assets to be rendered. If there are any transparent pixels in the sprite, they will be shown as black instead. That works good enough for most UI applications, but if we were to make a tool that draws custom graphics onto vanilla widgets, for example creature portraits or work detail icons, it would need to handle transparency in order to work.

[myk002]

Note that transparency is a function of what layer the texture is rendered to. I don't think it is related to the textures module itself.

Anything drawn to the interface layer will not support transparency.

Anything drawn to the anchor or map layers will.