Add custom attribute to CustomMaterial

[ludevik]

Check this post by @Evgeni_Popov:

Heat map on solid particles

uv1-uv6 are standard attributes, if they are provided on the mesh geometry they are automatically declared in the shaders (I think before 5.0.0 it was only uv1/uv2 that were automatically declared), so one should not redeclare them (see line 90): https://playground.babylonjs.com/#2LX5WU#18

Just remove the declaration of uv6, i’ve updated the PG for you.