This fixes an issue where the `use_masks` functions on layers
and groups returned the wrong value.
The issue was that the root group doesn't have this flag set
which then propagates to all the layers and groups.
To fix this we invert the `GP_LAYER_TREE_NODE_USE_MASKS` flag (now called `GP_LAYER_TREE_NODE_HIDE_MASKS`).
The API still uses the `use_masks` function.
Falk David
6a320524b9