GAL: disable glCopyBuffer for etnaviv vendor
glCopyBufferSubData crashes for vertex buffers on etnaviv (at least on Vivante GC7000L). As there is already a workaround in place for Intel, we can extend it for the etnaviv vendor.
This became apparent when rendering bigger/more complex boards that would crash KiCAD on MNT Reform. After this patch, it works fine.