tiff: Improve grayscale support

Only use PAL8 if palette is present, else use GRAY8 for pixfmt.

Instead of simulating a grayscale palette, use real grayscale pixels, if no
palette is actually defined.

Signed-off-by: Diego Elio Pettenò <flameeyes@flameeyes.eu>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>