Its members can be on copper layers, but the group itself isn't on any
layer.
Also fixes a bug where we were trying to clone TRIANGULATED_POLYGON::TRI
shapes as indexable sub-shapes. (The TRI only has indexes into its
parent, so cloning it will only result in segfaults down the line.)
Also fixes a bug where we weren't including copper items inside groups
when checking footprint net ties.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/15021
Also adds some debugging to try to catch dangling pointers.
Also adds a cache for group bounding boxes (which will be expensive
to calculate for large groups).
Fixes https://gitlab.com/kicad/code/kicad/issues/12875
Bitmaps are now identified by an enum class instead of by pointers.
Bitmap loading and caching is now handled by a class in common, and
we no longer compile most bitmaps into the binary, so there is no
longer a bitmaps static library.
Instead, bitmaps are archived to a .tar.gz file which is installed
in ${KICAD_DATA}/resources/images.tar.gz
The source PNGs are checked in to Git as the original CPP files were,
so that people can build without the required dependencies to convert
SVGs to PNGs.
Initial support is also added for dark theme icons, although this
is not yet exposed in the GUI.
Stubs are present for multi-resolution image resources, but this is
not fully-baked yet and could use some refinement.