Add renderer hook for DJGPP hardware SMZX mode.

[AliceLR]

The DJGPP port EGA renderer handled hardware SMZX mode in its set_video_mode function, which only gets called when initializing a renderer or creating a new window (as it corresponds to the SDL 1.x function of the same name). This patch adds an extra hook specifically for set_screen_mode to be called in update_screen prior to committing palette changes to the renderer.