Beginning GIF export

Beginning addition of an option to export animated GIFs
from the Open -> Slideshow dialog, after choosing the images.

Non-operable at this time, but a button has been added (and will
provide a hint to select 2 or more images, when clicked).

src/gifenc.h

@@ -0,0 +1,24 @@
+#ifndef GIFENC_H
+#define GIFENC_H
+
+#include <stdint.h>
+
+typedef struct ge_GIF {
+    uint16_t w, h;
+    int depth;
+    int fd;
+    int offset;
+    int nframes;
+    uint8_t *frame, *back;
+    uint32_t partial;
+    uint8_t buffer[0xFF];
+} ge_GIF;
+
+ge_GIF *ge_new_gif(
+    const char *fname, uint16_t width, uint16_t height,
+    uint8_t *palette, int depth, int loop
+);
+void ge_add_frame(ge_GIF *gif, uint16_t delay);
+void ge_close_gif(ge_GIF* gif);
+
+#endif /* GIFENC_H */