Previously weight paint wasn't hooked up to the "smooth" and "invert" modes.
I have added code so this is the case now. (And hooked up "smooth" for vertex paint as well)
What is not in this current patch is default keymap changes that I think we might need to discuss a bit.
My idea is that we bring these more in line with sculpt mode.
So holding shift will switch to the smooth brush and holding control will invert.
Currently in weight paint mode, shift and control is used for sampling weights, I think we should move these to S so we match it with the "sample color" short cut from other painting modes.
For vertex paint mode, only the shift key short cut needs to be added which will not "steal" any of the current shortcut keys in that mode.
This way I feel our short cuts will be more consistent between painting modes.