Replace MESH_BINDGROUP_1 with a ShaderDefVal

[nicopap]

Using MESH_BINDGROUP_1 as a define results duplicate code. And sometimes nested #ifdef in bevy shader code.

#ifdef MESH_BINDGROUP_1
@group(1) @binding(0)
var<storage> mesh: array<Mesh>;
#else // MESH_BINDGROUP_1
@group(2) @binding(0)
var<storage> mesh: array<Mesh>;
#endif // MESH_BINDGROUP_1

Duplicate code has been a source of programming bumps when developing rendering features. We should aim to reduce it.

What solution would you like?

Use a ShaderDefVal instead of a define for MESH_BINDGROUP.

The previous code would become:

@group(#{MESH_BINDGROUP}) @binding(0)
var<storage> mesh: array<Mesh>;

[JMS55]

I use the same pattern in bevy_solari, I'm in favor :)

[robtfm]

I didn’t do this originally because I didn’t want to force every pipeline author to include a def which would 95% of the time be the same thing (i.e. 2) …
As long as we have 2 as the default for when it’s not defined, it sounds good.

[JMS55]

MESH_BIND_GROUP_1 was removed #10485