MAINT-3585 FIXED Viewer Crashes when attempting to upload image.

The bug was fixed, the reasone of crash is following. The Core Flow view
contain another GL context and will not care about restoring a previous.
I restore context manually.
This path also contain a minor changes in another files.

All changes described here.

Сhange's for fix current bug.
indra/llwindow/llwindow.h
indra/llwindow/llwindowheadless.h
indra/llwindow/llwindowmacosx.h
indra/llwindow/llwindowsdl.h
indra/llwindow/llwindowwin32.h
indra/newview/lllocalbitmaps.cpp
indra/newview/llviewerdisplay.cpp
indra/newview/llviewerdisplay.h

Twice mUsage initialization (replace to forward initialization).
indra/llcharacter/lljointstate.h

Looks like condition should be befor memcopy call, otherwise - possible CRASH.
indra/llcommon/llmd5.cpp

Unused condition and variables.
indra/llmath/llsphere.cpp

Looks like should be under if otherwise - possible CRASH
indra\llprimitive\llmodel.cpp

Useless assert's.
indra/llrender/llrender.cpp
indra/newview/lldaycyclemanager.cpp