filters: Clear free_table flag on GP_FilterTablesInit()

Fixes abort when GP_FilterTablesFree() tries to free() stack allocated
tables structure.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: Add two point filters.

Add combined Brightness Contrast filter and Posterize filter.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Update/Add point filters docs.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: GP_Rotate.c: Fix whitespaces.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: Swap GP_MirrorH() and GP_MirrorV()

I've got it wrong when I was implementing them...

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: Rotate: Remove _Raw prototypes from header.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: Rotate: Remove the _Raw variants from API.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
filters: Remove unused GP_Noise.gen.c.t and GP_Point.gen.c.t

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
build: Update list of filter exported symbols.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: filters_python: Add rotation and symmetry filters.

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