Moved RGB fix inside ImageQt class

[radarhere]

Resolves #5266. A workaround suggested in that issue is to change the user code. This PR causes the original code mentioned there to work.

Pillow/src/PIL/ImageQt.py

Lines 203 to 205 in fdd8b68

	# Fix some strange bug that causes
	if im.mode == "RGB":
	im = im.convert("RGBA")

This 'strange bug' is because at https://doc.qt.io/qt-5/qimage.html

QImage::Format_RGB32... is stored using a 32-bit RGB format (0xffRRGGBB).

QImage is expecting the 4th channel to be populated with 0xff (255). Calling convert("RGBA") in Pillow does this.

Pillow/src/libImaging/Convert.c

Lines 421 to 431 in fdd8b68

	static void
	rgb2rgba(UINT8 *out, const UINT8 *in, int xsize) {
	int x;
	for (x = 0; x < xsize; x++) {
	*out++ = *in++;
	*out++ = *in++;
	*out++ = *in++;
	*out++ = 255;
	in++;
	}
	}

Without 255 values, as seen in the issue, the display in Qt is very washed out.

This PR just documents the fix slightly better, and moves it from toqpixmap() to _toqclass_helper(), so that it is also run when calling the ImageQt.ImageQt class (toqixmap() still works, since toqpixmap() calls toqimage(), which returns an instance of the ImageQt.ImageQt class).