[WIN32K]

- In NtGdiGetDIBitsInternal use a shared lock for the bitmaps and provide background colors when initializing the XLATEOBJ. This fixes mono bitmaps passed to GetDIBits. (no it does not fix pink icons in VLC)
- In BuildDIBPalette don't handle 15bpp, its not valid. and 16 bpp is 555, this is documented in MSDN.

svn path=/trunk/; revision=50920