Whether the SDL supports the touch panel?
I have the official 7" touchscreen hooked up to a Pi3, and it works (it sends SDL_FINGERDOWN events when you touch the screen, etc, for up to ten fingers at once). I can’t promise it’ll work with every touchscreen hooked up to random GPIO pins, but if it looks like a touchscreen to the Pi, it will probably look like one to SDL, too.
Is this with SDL2 from the Raspbian repository, or built from source (if it matters)?
I haven’t tried Raspbian’s, I only built from source.