Use u16 indices if possible in mesh primitives

[beicause]

Objective

Indices::U32 uses more memory and it's unnecessary if vertex count <= 65536 (most case). But all mesh primitives use Indices::U32

Solution

Use Indices::U16 if possible in mesh primitives.
I added Indices::with_capacity(), resize() and set() as helpers to create indices. Also added glam_bytemuck and used it in Capsule3dMeshBuilder.

Testing

Tested with the 2d_shapes and 3d_shapes examples.

[atlv24]

i believe the mesh merge op does not work on meshes of distinct index types. just something to keep in mind, this might break some use cases.

[beicause]

bevy/crates/bevy_mesh/src/mesh.rs

Lines 1044 to 1047 in c5c7598

	// Extend indices of `self` with indices of `other`.
	if let (Some(indices), Some(other_indices)) = (self.indices_mut(), other.indices()) {
	indices.extend(other_indices.iter().map(|i| (i + index_offset) as u32));
	}

bevy/crates/bevy_mesh/src/index.rs

Line 118 in c5c7598

fn extend<T: IntoIterator<Item = u32>>(&mut self, iter: T) {

If I understand correctly, merge op can extend u16 to u32 if it will overflow so this doesn't break.