Fix shader_material_glsl example after #9254

[superdump]

Objective

• Fix shader_material_glsl example

Solution

• Expose the PER_OBJECT_BUFFER_BATCH_SIZE shader def through the default MeshPipeline specialization.
• Make use of it in the custom_material.vert shader to access the mesh binding.

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Changelog

• Added: Exposed the PER_OBJECT_BUFFER_BATCH_SIZE shader def through the default MeshPipeline specialization to use in custom shaders not using bevy_pbr::mesh_bindings that still want to use the mesh binding in some way.

[robtfm]

shame we have to reintroduce the shader def in the mesh pipeline but hopefully it'll be temporary.