Lightness and Gradient modes for brush textures

This merge request adds lightness and gradient options in brush textures.

Lightness option for textures uses the texture as a lightness map with the selected color. Allows texture brushes that paint the texture much more vibrantly.

Gradient option for textures maps the texture value to the value in the selected gradient. This can make even more vibrant textures than the lightness mode.

For both options, the Strength modifier controls how much the texture appears, compared to the selected paint color. At strength = 100%, the texture is drawn at full strength. At strength = 0%, the texture does not appear at all, and only the paint is drawn. Values in between create a mix between the paint and the texture.

Includes gradient features from MR 369

This is a partial MR from MR 348