User side: * Preview for DynamicPaint should keep the same behavior (for now). Weight preview should be somawhat quicker, though. * Preview for WeightVG modifiers is only active in WeightPaint mode, and if the affected vgroup is the active one. * Last active preview modifier in stack wins! Note: that modifier preview topic is yet to be further refined, quite raw/incomplete for now. Dev side: * In draw code, renamed DRAW_DYNAMIC_PAINT_PREVIEW flag to DRAW_MODIFIERS_PREVIEW * Removed use of MOD_DPAINT_PREVIEW_READY in DynamicPaint code (seems unecessary, and if it was, should be of more general scope). * Added eModifierTypeFlag_UsesPreview to ModifierTypeFlag, for modifiers that can generate some preview data. * Added three new modifier funcs, to handle preview modifiers in draw code / mod stack. * For weights preview: added the generic DM_update_weight_mcol func, which can update WEIGHT_MCOL layer with either a given array of weights (currently used by DynamicPaint only), or from current active vgroup(s). So now, draw code is fully generic (i.e. no more modifier-type checking in it). Mod stack code is generic to some extent, but will need more work.