Remove unused ImagingConvertInPlace

[radarhere]

im.putalpha() is the only place where C's setmode() is called.

Pillow/src/PIL/Image.py

Lines 2050 to 2052 in 3220aca

	mode = getmodebase(self.mode) + "A"
	try:
	self.im.setmode(mode)

This is only trying to convert the image to LA, PA or RGBA.

However, the only operation setmode() can actually succeed at is converting RGB or RGBX to RGBA.

Pillow/src/_imaging.c

Lines 2079 to 2090 in 3220aca

	if (im->mode == mode) {
	; /* same mode; always succeeds */
	} else if (isRGB(im->mode) && isRGB(mode)) {
	/* color to color */
	im->mode = mode;
	im->bands = modelen;
	if (mode == IMAGING_MODE_RGBA) {
	(void)ImagingFillBand(im, 3, 255);
	}
	} else {
	/* trying doing an in-place conversion */
	if (!ImagingConvertInPlace(im, mode)) {

Pillow/src/libImaging/Convert.c

Lines 1739 to 1748 in 3220aca

	ImagingConvertInPlace(Imaging imIn, const ModeID mode) {
	ImagingSectionCookie cookie;
	ImagingShuffler convert;
	int y;
	/* limited support for inplace conversion */
	if (imIn->mode == IMAGING_MODE_L && mode == IMAGING_MODE_1) {
	convert = l2bit;
	} else if (imIn->mode == IMAGING_MODE_1 && mode == IMAGING_MODE_L) {
	convert = bit2l;

ImagingConvertInPlace() isn't able to convert anything to LA, PA or RGBA.

So this PR

• removes ImagingConvertInPlace(). setmode() is the only place where ImagingConvertInPlace() is called.
• simplifies setmode() to more clearly only convert RGB or RGBX images to RGBA. I've also renamed it to setalpha().

[akx]

I had been looking at removing ImagingConvertInPlace (in a bid to see if the shuffler functions could be implemented in a faster way) but didn't dare go through with it.

Some thoughts within though :)

[akx]

This comment is a little stale now?

[radarhere]

Is it? The method is still modifying the mode of an image in place. It's more specific now, is all.

[akx]

I suppose...

Should this function know how to do P-to-PA and L-to-LA too?

[radarhere]

The thought occurred to me. It is a bit different to RGB, since RGB already has the same pixelsize as RGBA.

Do you mind if that is a follow-up PR? I would rather tidy things up first.

[hugovk]

I'll click the rebase button to test automerge (#9126) on a non-docs PR: