Direct3D 9 dictates that caps.NumSimultaneousRTs must always be at least 1,
which is to say that Direct3D 9 level hardware must always support render
targets.
(caps.NumSimultaneousRTs is meant to show if you can draw to multiple render
targets in a single draw call.)
We had already hardcoded SDL_RENDERER_TARGETTEXTURE as available earlier in
the function anyhow.
Fixes#4781.