[oden] Writable Textures and re-work TS API

Now we return Texture objects to make things a little bit more
type-safe, at the cost of a small allocation (I hope!)

types/graphics-core.d.ts

@@ -1,7 +1,9 @@
 // These are the functions exposed by the native graphics module.
 //
 export function cls(r: number, g: number, b: number);
+
 export function print(msg: string);
+
 export function spr(
   x: number,
   y: number,
@@ -12,8 +14,20 @@ export function spr(
   sw: number,
   sh: number
 );
+
 export function create_texture(
   buffer: ArrayBuffer,
   label: string | undefined
 ): number;
+
 export function use_texture(id: number);
+
+export function create_writable_texture(
+  width: number,
+  height: number,
+  label: string | undefined
+): number;
+
+export function write_to_screen();
+
+export function write_to_texture(id: number);