Blender Git Loki

Fix T68719 Viewport: Wrong material index when switching between view mode

This was due to the GPUIndexBuf ranges only computed when requesting
the triangles indices. If the tris were already calculated, the new shading
batches would never have the GPUIndexBuf ranges and instead use the full
triangle index buffer.

So since this only happen when shading data does not match, we just save
the previous GPUIndexBuf ranges and reuse them for the new batch.

This patch is a bit of a hack on top of a hack but it works fine.