fix picking in webgl1 for isosurface and slice

src/mol-gl/shader/image.frag.ts

@@ -104,7 +104,7 @@ void main() {
         #elif defined(dRenderVariant_pickInstance)
             gl_FragColor = vec4(encodeFloatRGB(vInstance), 1.0);
         #elif defined(dRenderVariant_pickGroup)
-            float group = texture2D(tGroupTex, vUv).r;
+            float group = texture2D(tGroupTex, vUv).a;
             gl_FragColor = vec4(encodeFloatRGB(group), 1.0);
         #endif
     #elif defined(dRenderVariant_depth)
@@ -119,7 +119,7 @@ void main() {
         gl_FragColor = imageData;
         gl_FragColor.a *= uAlpha;
 
-        float group = texture2D(tGroupTex, vUv).r;
+        float group = texture2D(tGroupTex, vUv).a;
         float vMarker = readFromTexture(tMarker, vInstance * float(uGroupCount) + group, uMarkerTexDim).a;
         #include apply_marker_color
         #include apply_fog

src/mol-repr/volume/isosurface.ts

@@ -61,6 +61,7 @@ export async function createVolumeIsosurfaceMesh(ctx: VisualContext, volume: Vol
 
     const transform = Grid.getGridToCartesianTransform(volume.grid);
     Mesh.transform(surface, transform);
+    if (ctx.webgl && !ctx.webgl.isWebGL2) Mesh.uniformTriangleGroup(surface);
 
     surface.setBoundingSphere(Volume.getBoundingSphere(volume));