MultiEffect padding API

MultiEffect should have some helpers for padding the source item so that blur & shadow can fit into the effect item without requiring user to manually add padding already into the source item.

Currently MultiEffect contains "bool paddingEnabled" and "QRectF paddingRect" properties for this. One problem is that paddingEnabled enables also automatic padding calculation where padding amount depends on blurMax and blurMultiplier so user can't set exact padding / item size.

Idea:

• Remove "paddingEnabled" as it is not needed. Empty paddingRect(0,0,0,0) does the same thing.
• Add "autoPadding" (default true) which controls if padding is automatically adjusted.
  • When false, padding is exactly the paddingRect.
  • When true (and blurEnabled || shadowEnabled), padding increases based on blurMax and blurMultiplier. Possible paddingRect is added on top so it can be made to work with different shadow positions.