Weights view improvements

[erikari]

• Add Weight Overlay opacity to control how much of the original image is seen through Overlay.

• Add pen pressure sensitivity (for Wacom and such)

• Spine could read normal or depth map images to auto generate the weights

Reference thread: http://esotericsoftware.com/forum/Could-there-be-improvements-for-the-Weight-Painting-13441

[misaki-eymard]

+1 regarding adding pen pressure sensitivity: https://esotericsoftware.com/forum/d/23843-pen-pressure-support-progress

[DASilverStraw]

+1 Add pen pressure sensitivity.