- fix issue:

- fix issue:
    abort() called
    *** Terminating app due to uncaught exception 'NSInvalidArgumentException'
    reason: '*** -blueComponent not defined for the NSColor NSCustomColorSpace Generic RGB colorspace 0.5 0.5 0.5 1; need to first convert colorspace.'
- remove superfluous color conversions; according to NSGradient docs, any needed colorspace conversions of params are automatic
- use [NSColor colorWithAlphaComponent] to force alpha
- change [NSColor colorWithSRGBRed] to [NSColor colorWithDeviceRed] for 10.6 API compatibilty
- [cosmetic] rename parm withEndColor -> endColor
- [leak] release NSGradient

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@4972 b64f7644-9d1e-0410-96f1-a4d463321fa5