Load and process images in the background about imv

[eXeC64]

Possible design:

/* set the url/filepath */
void imv_loader_set_path(struct imv_loader *ldr, const char *path);

/* returns ptr if a new texture for the current image is available, NULL if nothing new */
FIBITMAP *imv_loader_get_texture(struct imv_loader *ldr);

/* returns 1 if an image failed to load and sets out_path to the path that failed */
int imv_loader_did_error(struct imv_loader *ldr, const char **out_path);

/* ask for the next frame to be loaded ASAP */
void imv_loader_next_frame(struct imv_loader *ldr);

/* tell loader dt time has passed, if it's time for a new frame, pass it back to us */
void imv_loader_time_passed(struct imv_loader *ldr, double dt);

[eXeC64]

Completed.