loaders: Add save_ptypes to GP_Loader structure

Add save_ptypes, an array of pixel types supported by the Loader's
Save() method.

Note that these arrays does _NOT_ describe all formats that could be
saved by the loader because most of the loaders use LineConvert() for
trivial conversions (i.e. RGB888 to BRG888 to xRGB888 etc.)

Convinient interface to query if particular pixel type coud be saved by
particular loader will be added later.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders; GP_Loader: Fix typo in loaders table.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: Rename Match functions + constify + doc

* Rename MatchExtension to LoaderByFilename
  and MatchSignature to LoaderBySignature

* Constify GP_Loader arguments where possible

* Update loaders docs

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