Optimize palette lookup
I see palette uses texture2D texture now. What about change it to texture1D? While wasm4 support only 4 palette slots (2-bit palette) I guess it could be even simpler approach and pass this four colours via 4 uniforms and use something like this:
```glsl
// palette
uniform vec2 p0;
uniform vec2 p1;
uniform vec2 p2;
uniform vec2 p3;

uniform sampler2D framebuffer;
varying vec2 framebufferCoord;

vec4 lookup(float img, vec4 p0, vec4 p1, vec4 p2, vec4 p3) {
   // not sure about implementation. Just basic idea
   // instead "mix" it could be "step"
   vec4 p;
   p = mix(p0, p1, mix(0.0,       1.0 / 3.0, img));
   p = mix(p,  p2, mix(1.0 / 3.0, 2.0 / 3.0, img));
   p = mix(p,  p3, mix(2.0 / 3.0,       1.0, img));
   return p;
}

void main() {
  gl_FragColor = lookup(texture2D(framebuffer, framebufferCoord).r, p0, p1, p2, p3);
}
```