Bf/powerspectrum

Closes #189

See merge request fsl/fsleyes/fsleyes!224

between ComplexPowerSpectrumSeries and Imaginary/Real/Magnitude - there is no
guarantee that CPSS.getData will be called before the others, so we can't
pre-calculate and cache the result. Currently just duplicate calculations for
each series, as the performance shouldn't really be a problem, and it keeps
the code fairly clean.

- data is retrieved, fourier transformed, then cached.

to be customised, while still taking advantage of the voxel cache.

it this way

Enh/volume lighting

See merge request fsl/fsleyes/fsleyes!222

as gl14 POW instruction seems to behave differently to GL21 pow function

why blending differs between 14 and 21. It looks like the POW instruction
simply does not produce the same results as the glsl pow function

wxagg will interfere with subsequent headless jobs

fsleyes.gl. More to come.

routines. Change meaning of GL_VERSION field, and add GL_COMPATIBILITY
field. Add new glIsSoftwareRenderer function to test for software GL renderer.