gbagfx: Various fixes and improvements

Fix num_colors exceeding amount of colors in palette leading to use of uninitialized colors
Handle png -> pal conversion (aka extracting JASC palettes from png)
Allow JASC pal files to be used as -palette argument when converting from GBA to PNG
Allow using only file extension as output to reuse entire path from input before extension

tools/gbagfx/util.c

@@ -47,6 +47,13 @@ char *GetFileExtension(char *path)
 	while (extension > path && *extension != '.')
 		extension--;
 
+	return extension;
+}
+
+char *GetFileExtensionAfterDot(char *path)
+{
+	char *extension = GetFileExtension(path);
+
 	if (extension == path)
 		return NULL;