Removed all mention of 'softwareMode'. SceneOpts.performance now has

fsl/data/strings.py

fsl/fsleyes/controls/overlaydisplaypanel.py

fsl/fsleyes/controls/overlaydisplaytoolbar.py

fsl/fsleyes/displaycontext/canvasopts.py

fsl/fsleyes/displaycontext/display.py

fsl/fsleyes/displaycontext/sceneopts.py

fsl/fsleyes/gl/gl14/gllabel_funcs.py

fsl/fsleyes/gl/gl14/gllinevector_funcs.py

fsl/fsleyes/gl/gl14/glrgbvector_funcs.py

fsl/fsleyes/gl/gl14/glvolume_funcs.py

fsl/fsleyes/gl/gl21/gllabel_funcs.py

fsl/fsleyes/gl/gl21/gllinevector_funcs.py

fsl/fsleyes/gl/gl21/glrgbvector_funcs.py

fsl/fsleyes/gl/gl21/glvolume_funcs.py

fsl/fsleyes/gl/gllabel.py

fsl/fsleyes/gl/gllinevector.py

fsl/fsleyes/gl/glmask.py

fsl/fsleyes/gl/glvector.py

fsl/fsleyes/gl/glvolume.py

fsl/fsleyes/gl/slicecanvas.py