loaders: JPG: Remove stub for removed function.

Remove GP_OpenJPG() stub for removed function.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: ResizeLinearLF: Rewrite.

Now faster, simpler and working for 16 bit Grayscale.

The code needs to be a cleaned a bit later though.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>