Don't go nulling out curShader every flush

I don't know how I missed that the last time I went diving
down this rabbit hole, but by clearing that any time we flush,
we always assume we've got a shiny new shader when we then use
it again.

And in particular, the symptom I ran against was that it would
have forgotten the shader by the time it got around to REALLY
swapping out for a different one, resulting in the bug I fixed
back in faf27d6a88 rearing its ugly head again because now I
had two flushes worth of stuff with an IndexedTris-based shader.
Whitespace