webp: Make it a module and unbundle libwebp thirdparty files

Note that there are two Godot-specific changes made to libwebp
for the javascript/HTML5 platform. They are documented in the
README.md.

modules/webp/config.py

@@ -0,0 +1,6 @@
+
+def can_build(platform):
+	return True
+
+def configure(env):
+	pass