renderer: don't hard-require PL_FMT_CAP_BLITTABLE

Some formats don't support this cap, and we don't strictly need it
(since we can work around it using pl_tex_blit_raster).

I plan on perhaps refactoring PL_FMT_CAP_BLITTABLE into
PL_FMT_CAP_BLITTABLE_1D_3D in the future, but for now, this fix is
needed.