Waveform: add some headroom for values > 1.0

Keep in mind that under some circumstances these values get clipped, for
example when enabling lcms2 in the prefs. Took me quite some time ...
opencl: set locale to "C" during initialization. fixes #9467
Merge branch 'master' of ssh://github.com/darktable-org/darktable
Merge remote-tracking branch 'upstream/master' into gtk3-porting