When combining samples, take fp16 rounding into account.

This makes us somewhat more conservative in combining samples;
when we are near the lower/right edges of the image, we are starting
to get close to 1.0, and fp16 just doesn't have enough precision
to give us the 6 or 8 bits of subpixel precision we want (it is
hardly enough to address individual pixels!). In particular, this
can affect zooming with ResampleEffect, as reported by Christophe
Thommeret.

This does not fix all cases (especially not non-power-of-two cases);
for that, we will probably need to be able to fall back to fp32
when we detect fp16 doesn't work well.