core: Move GP_Fill to core.

Sometimes loaders or other parts of the library needs to prefill a
context with some pixel value. Now they can use GP_Fill since it has
been moved to core (also optimized slightly).

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
spiv: Two fixes for the slideshow timer.

The -s parameter takes time in seconds (floating point value).

If the timer kicked in but the previous image is not loaded yet the
timer will return immediately and retry after 20 ms.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders; BMP_RLE: Whitespace cleanup.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: BMP: Make use of GP_Fill.

As the GP_Fill() is now in core, make use of it.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
backends: Add GP_BackendRemTimer()

Add missing backends wrapper for removing timers from backend timers
queue + update backends docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
spiv: Key S now can stop and restart slideshow.

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