GPencil: Add automatic sorting for Mask List

Now the masking is sorted in the same order of the layers. The order is recalculated when any layer is created, deleted or moved.

GPencil: Move mask functions to BKE and fix delete layer

When delete a layer, all layers using this layer as mask must be updated.

GPencil: Refactor: Fix completely hard stroke not being 100% hard

This was a problem with intensily lit gpencil objects.

GPencil: Refactor: Remove the hardcoded sRGB final transform for viewport

Now viewport and render has the same look and colormanagement applied to it

GPencil: Refactor: Fix blend modes broken

GPencil: Refactor: Add per mask inversion capability

With this commit, masks can be inverted per layer. Inversion means the
mask becomes subtractive instead of additive. The base visibility does not
change depending on the first type (i.e: if ther is nothing in the mask
layer, nothing will be masked regardless of the mask type).

GPencil: Change Stroke Order tooltip

GPencil: Reset Mask list index when remove first element

GPencil: Replace Add mask with popover menu

Also removed iinternal property used.

Merge branch 'greasepencil-object' into greasepencil-refactor

Conflicts:
source/blender/draw/engines/gpencil/gpencil_engine.h
source/blender/draw/engines/overlay/overlay_extra.c
source/blender/draw/intern/draw_manager_exec.c

Merge branch 'master' into greasepencil-object

GPencil: Change icons for Invert Mask: ON/OFF

GPencil: Implement Stroke Hardeness

Still pending the aspect ratio

GPencil: Fix typo error in struct name for write

This produced errors loading the mask list.

GPencil: Fix merge layers operator to merge Mask list

GPencil: Add new Hide and Invert icons to Masks

GPencil: Add limit control for number of masking layers

Merge branch 'greasepencil-object' into greasepencil-refactor

Merge branch 'master' into greasepencil-object

GPencil: Refactor: Add implementation of new mask list system

This system makes it possible to have up to 256 layers in a GPencil Object
that can be used as masks. User can use more than 256 layers and use mask
on them but the masking one must be from one of the first 256.

This commit does not include optimizations.