Mnt/highdpi

See merge request fsl/fsleyes/fsleyes!338

settings being removed and added back in

TEST: Ignore minor artifact when running off-screen on macOS (using conda-forge mesalib)

See merge request fsl/fsleyes/fsleyes!336

conda-forge mesalib)

Lightbox redesign

See merge request fsl/fsleyes/fsleyes!334

by using a weakref. Also periodically trigger garbage collection to keep
unnecessary mem usage down.

a web browser - instead, the kernel connection file is printed to stdout

conversion. Adjust slice spacing when Z axis changes - e.g. if ssp is at min,
and Z axis channges from one with many voxels to one with few voxels, we
want the slice spacing to be adjusted such that there is one slice per voxel

overlays. This will resolve occasional issues of time series / power spectra
not appearing when opening melodic mode

even for shaders which don't use it (and use tex coord arrays for vertices instead)

spacing for per-overlay slice spacing value

exposed in render command

options. Used to initialise LightBoxOpts.zrange to be centred around
the current z location

more easily set/persisted

specify initial/default location/size