sRGBtoRGB and RGBtosRGB do not rescale, resulting in inaccurate conversions

[FallingPineapples]

Unlike other colorspace conversions, sRGBtoRGB and RGBtosRGB do not rescale the input or output between imager's default 0-1 range and CImg's 0-255 range; since sRGB is not linear, this results in quite inaccurate conversions. Both input and output rescalings would be required for accurate conversion.

imager/src/colourspace.cpp

Lines 81 to 97 in 13ec8a0

	//' @describeIn imager.colourspaces RGB to sRGB conversion
	//' @export
	// [[Rcpp::export]]
	NumericVector RGBtosRGB(NumericVector im) {
	CId img = as<CId >(im);
	img.RGBtosRGB();
	return wrap(img);
	}
	//' @describeIn imager.colourspaces sRGB to RGB conversion
	//' @export
	// [[Rcpp::export]]
	NumericVector sRGBtoRGB(NumericVector im) {
	CId img = as<CId >(im);
	img.sRGBtoRGB();
	return wrap(img);
	}

For example, converting white in RGB to sRGB results in values well out of range:

c(1,1,1) %>% as.cimg %>% RGBtosRGB %>% str
# 'cimg' num [1, 1, 1, 1:3] 12.7 12.7 12.7