Ability for UI to use external masks

[qreeves]

Textures can generate an alpha mask which can be used by the UI for hit detection, but these can't be used on elements other than the originating texture. Add the ability to reference a separate mask for both rendering and hit detection to any UI element.

Requirements:

• HUD shader that takes a second texture to use a mask while rendering.
• UI code to check the mask of a separate texture for hit detection.
• Ability for mask checks to be scaled to the dimensions of the element.