Blender Git Loki

Fix T38773: Inconsistent conversion of colors and float values in
Blender Internal shader nodes.

All nodes now use the `nodestack_get_vec` function to convert whatever
input values are connected to the desired type, instead of using the
stack values directly regardless of what they contain.

Note that this might break old node setups that have been adapted to the
faulty behavior, but as discussed in the bug report the 2.70 version is
the right time to fix this.