`fgain` in `CLIPPED` combine mode

[hgao-astro]

swarp/src/coadd.c

Lines 1368 to 1369 in bf4f496

	sigmaeff = sqrtf(sumw + f1f2o2*(1.0/infields[o1]->fgain
	+ 1.0/infields[o2]->fgain)),

swarp/src/coadd.c

Line 1411 in bf4f496

float sigmaeff = sqrtf(*(pixwstack+i)+amu/infields[o]->fgain);

The doc says that setting the gain to 0 means infinity. However, this seems not properly implemented in the clipping, where a zero gain leads to infinite sigma and thus no clipping.

My use case is to provide SWARP with absolute weight maps computed as inverse variance and to turn off RESCALE_WEIGHTS. The variance already includes both background noise and Poisson noise of the sources. To avoid double counting, I need the second term to be zero and the gain to be infinitely large. I can manually set the gain to a very large value in the header keywords of resampled images before coadding. But it would be nice to see that SWarp can handle this.