buffer: use internal floorf/ceilf implementations

This avoids both function call overhead and extra checks for values that
are outside MININT/MAXINT range, for the uses inside GeglBuffer we only
care about coordinates that are representable as integers - thus these
inline-able shortcut versions are good enough for us.

The speedup seems to consistently make all samplers about 10% faster
according to the tests in perf/
parent e0d79197
Pipeline #82503 failed with stages
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