Image texture node alpha premultiply parameter

[Tugcga]

Add parameter to define alpha premultiply for ImageTexture shader node. In blender it can be selected from 4 variants:

• Streight. Output alpha and color multiplied to the alpha
• Premultiplied. Works similar to the previous one (what the difference?)
• Channel Packed. Output alpha and color without multiplication
• None. Ignore alpha, set it 1.0 May be for simplicity only two items should be implemented.

[Tugcga]

Will be available in 2.1.2 update