Dither using jpeg not png to avoid odd coloring

[jtomson]

For some reason when imgproxy converts a source image from heic to png prior to invoking the dither test.py, the resulting image is much more muted and yellow than expected when running the image locally.

This may have something to do with the PNG options used by default

imgproxy/vips/vips.go

Lines 372 to 376 in 44fdf48

	switch imgtype {
	case imagetype.JPEG:
	err = C.vips_jpegsave_go(img.VipsImage, &ptr, &imgsize, C.int(quality), vipsConf.JpegProgressive)
	case imagetype.PNG:
	err = C.vips_pngsave_go(img.VipsImage, &ptr, &imgsize, vipsConf.PngInterlaced, vipsConf.PngQuantize, vipsConf.PngQuantizationColors)

Working around this by converting from source -> jpg before invoking the dither test.py, which does the trick.

[jtomson]

Closing in favor of #47