Thumbnail cache file format depending on existance of alpha layer

Use PNG if image has alpha layer, JPG if not. Fixes issue #209

Makefile

@@ -1,4 +1,4 @@
-VERSION   := git-20150518
+VERSION   := git-20150722
 
 PREFIX    := /usr/local
 MANPREFIX := $(PREFIX)/share/man

thumbs.c

@@ -48,9 +48,9 @@ char* tns_cache_filepath(const char *filepath)
 	
 	if (strncmp(filepath, cache_dir, strlen(cache_dir)) != 0) {
 		/* don't cache images inside the cache directory! */
-		len = strlen(cache_dir) + strlen(filepath) + 6;
+		len = strlen(cache_dir) + strlen(filepath) + 2;
 		cfile = (char*) s_malloc(len);
-		snprintf(cfile, len, "%s/%s.jpg", cache_dir, filepath + 1);
+		snprintf(cfile, len, "%s/%s", cache_dir, filepath + 1);
 	}
 	return cfile;
 }
@@ -101,8 +101,12 @@ void tns_cache_write(Imlib_Image im, const char *filepath, bool force)
 			}
 			if (err == 0) {
 				imlib_context_set_image(im);
-				imlib_image_set_format("jpg");
-				imlib_image_attach_data_value("quality", NULL, 90, NULL);
+				if (imlib_image_has_alpha()) {
+					imlib_image_set_format("png");
+				} else {
+					imlib_image_set_format("jpg");
+					imlib_image_attach_data_value("quality", NULL, 90, NULL);
+				}
 				imlib_save_image_with_error_return(cfile, &err);
 			}
 			if (err == 0) {