FreeRDP RDPGFX ResetGraphics race leads to use-after-free in SDL client (sdl->primary)

FreeRDP is a free implementation of the Remote Desktop Protocol. Prior to 3.20.1, a race condition between the RDPGFX dynamic virtual channel thread and the SDL render thread leads to a heap use-after-free. Specifically, an escaped pointer to sdl->primary (SDL_Surface) is accessed after it has been freed during RDPGFX ResetGraphics handling. This vulnerability is fixed in 3.20.1.