feat: Skip texture copying for image elements that haven't changed (#6)

* Skip image texture copying for <img> elements

* Add changeset

.changeset/lazy-onions-tan.md

···
+---
+'frag-canvas': patch
+---
+
+Skip loading `<img>` as texture on every frame and skip on repeats

src/frag-canvas-element.ts

···
   return [year, month, day, time] as const;
 };
 
+const isImageElement = (tex: TexImageSource): tex is HTMLImageElement =>
+  (tex as Element).tagName === 'IMG';
+
 const preprocessShader = (source: string) => {
   let header = '';
   let output = source.trim();
···
 
   let frameCount = 0;
   let prevTimestamp: DOMHighResTimeStamp;
+  let prevSource: string | null;
 
   const state = {
     draw(source: TexImageSource, timestamp: DOMHighResTimeStamp) {
···
 
       gl.useProgram(program);
 
-      if (source) {
+      if (isImageElement(source)) {
+        if (source.complete) {
+          const { currentSrc } = source;
+          if (prevSource !== currentSrc) {
+            prevSource = currentSrc;
+            gl.texImage2D(
+              gl.TEXTURE_2D,
+              0,
+              gl.RGBA,
+              gl.RGBA,
+              gl.UNSIGNED_BYTE,
+              source
+            );
+          }
+        }
+      } else if (source) {
+        prevSource = null;
         gl.texImage2D(
           gl.TEXTURE_2D,
           0,
···
         if (iChannelResolution)
           gl.uniform3fv(iChannelResolution, [width, height, 0]);
       } else {
+        prevSource = null;
         if (iChannelResolution) gl.uniform3fv(iChannelResolution, [0, 0, 0]);
       }