For DC initialization we check the thread ID to assign a normal
or cached DC to the corresponding WinSalGraphics variable.
The cached DC has a usage count, as there are some limits on
cached DCs count (DCX_CACHE).

But for the WinSalGraphics DC init and release the variable just
matters for the accounting, and generally which thread is doing
the calls: the non-main thread always has to relay them to the
main application thread.

Since we're releasing all WinSalGraphics in ~WinSalFrame and do
all release and re-init in ImplSetParentFrame, there is no way
to correspond the thread ID to the WinSalGraphics variable.

So this drops the wrong assertions based on the WinSalGraphics
variables and renames the GETDC message to GETCACHEDDC to make
usage of a cached DC (DCX_CACHE) more obvious.
As a consequence of the different release DC handling this also
fixes the accounting of the cached DCs, wich was broken in the
initial fix; commit c15ea73f

Change-Id: I11ce52a1b4005f26567f92588437fa37bf227a2e
Reviewed-on: https://gerrit.libreoffice.org/51318Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>