backends: Linux FB: Declare local function as static.

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

Adds LineConvert API for trivial pixel conversions
(i.e. RGB888 to BGR888 and so).

Make use of it in BMP and JPG loaders.
(more to come)

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