Fix startup world init and inverted sand gravity

src/App.ts

@@ -31,6 +31,7 @@ export default new (class App {
 
     constructor() {
         this.initScenes();
+        this.resetRenderer();
 
         window.addEventListener("resize", () => this.resize());
 

src/shaders/sandPhysics.frag

@@ -36,7 +36,7 @@ void main() {
     float behaviors[4];
     float densities[4];
     for (int i = 0; i < 4; i++) {
-        ivec2 p = blockBase + ivec2(i % 2, i / 2);
+        ivec2 p = blockBase + ivec2(i % 2, 1 - i / 2);
         cells[i] = texelFetch(uWorld, p, 0);
         vec4 props = texelFetch(uMaterialProps, ivec2(int(cells[i].r), 0), 0);
         behaviors[i] = props.r;