Ensure image is opaque after converting P to PA with RGB palette

[radarhere]

When converting P to PA, Pillow currently sets the A value from the palette.

Pillow/src/libImaging/Convert.c

Lines 1029 to 1039 in cac305f

	static void
	p2pa(UINT8 *out, const UINT8 *in, int xsize, const UINT8 *palette) {
	int x;
	for (x = 0; x < xsize; x++, in++) {
	const UINT8 *rgba = &palette[in[0]];
	*out++ = in[0];
	*out++ = in[0];
	*out++ = in[0];
	*out++ = rgba[3];
	}
	}

If the palette is in RGBA mode, that makes sense. However, if the palette is in RGB mode, then that doesn't make sense, and leads to this.

from PIL import Image
Image.open("Tests/images/hopper.png").convert("P").convert("PA").save("out.tiff")

out.tiff.zip

Instead, this PR checks the palette mode, and fills the alpha channel with 255 if the palette is RGB.