fixed hdc initialization into wf_begin_paint so the region invalidation into wf_end_p...

fixed hdc initialization into wf_begin_paint so the region invalidation into wf_end_paint applies to the same hdc
the objective is that only the updated region(s) are refreshed instead of the whole screen
it's not much a problem for normal FreeRDP usage but becomes a serious issue if the display is processed, forwarded and rendered remotely, as done by web gateways such as Guacamole, FreeRDP-WebConnect and Myrtille (I'm the author of the latter)