[Ohrrpgce] SVN: teeemcee/12345 gfx_sdl2: fix mouse drags off the window not working

[subversion at HamsterRepublic.com]

teeemcee
2021-09-04 08:05:28 -0700 (Sat, 04 Sep 2021)
344
gfx_sdl2: fix mouse drags off the window not working

gfx_sdl2 mouse input went dead (with mouse buttons unpressed) as soon as the
mouse left the window.
Need to explicitly capture the mouse, unlike SDL 1.2

This caused problems for some games (like Katya's Abyss) where you could drag
off the window to scroll, and also for the sprite editors.
---
U   wip/allmodex.bas
U   wip/gfx.bi
U   wip/gfx_sdl2.bas