[0.4.9][DESK] Fix GetDC/ReleaseDC error management

[0.4.9][DESK] Fix GetDC/ReleaseDC error management

Fix missing ReleaseDC related to the spectrum (color depth)

We leaked one device context for each bpp change within the
desk.cpl-session, which gave the following logging when closing desk.cpl:
(win32ss/user/ntuser/windc.c:749) err: [00060138] GetDC() without ReleaseDC()!
1 time for each leak.

Partial backport of (#2707)
Fix picked from 0.4.15-dev-2867-g d635ce0cc0336a9b77d243ddce946876f48ec4f6