fix webgl error in volume-streaming behavior

- don't use gpu mc for small volumes
- return empty texture-mesh for volumes of size 1 or 0

src/mol-repr/volume/isosurface.ts

@@ -43,6 +43,10 @@ function gpuSupport(webgl: WebGLContext) {
 const Padding = 1;
 
 function suitableForGpu(volume: Volume, webgl: WebGLContext) {
+    // small volumes are about as fast or faster on CPU vs integrated GPU
+    if (volume.grid.cells.data.length < Math.pow(10, 3)) return false;
+    // the GPU is much more memory contraint, especially true for integrated GPUs,
+    // fallback to CPU for large volumes
     const gridDim = volume.grid.cells.space.dimensions as Vec3;
     const { powerOfTwoSize } = getVolumeTexture2dLayout(gridDim, Padding);
     return powerOfTwoSize <= webgl.maxTextureSize / 2;
@@ -131,7 +135,6 @@ namespace VolumeIsosurfaceTexture {
     export function get(volume: Volume, webgl: WebGLContext) {
         const { resources } = webgl;
 
-
         const transform = Grid.getGridToCartesianTransform(volume.grid);
         const gridDimension = Vec3.clone(volume.grid.cells.space.dimensions as Vec3);
         const { width, height, powerOfTwoSize: texDim } = getVolumeTexture2dLayout(gridDimension, Padding);
@@ -169,6 +172,10 @@ namespace VolumeIsosurfaceTexture {
 async function createVolumeIsosurfaceTextureMesh(ctx: VisualContext, volume: Volume, theme: Theme, props: VolumeIsosurfaceProps, textureMesh?: TextureMesh) {
     if (!ctx.webgl) throw new Error('webgl context required to create volume isosurface texture-mesh');
 
+    if (volume.grid.cells.data.length <= 1) {
+        return TextureMesh.createEmpty(textureMesh);
+    }
+
     const { max, min } = volume.grid.stats;
     const diff = max - min;
     const value = Volume.IsoValue.toAbsolute(props.isoValue, volume.grid.stats).absoluteValue;