Improve render logging by add echo to render script

Add echo(<scadline>) to the render system so that the output to std in CI shows which frames are being rendered. This will help to identify failures and whether particular frames are slow.

Before:

Exporting /tmp/scadfile.scad...
WARNING: Viewall and autocenter disabled in favor of $vp*
Compiling design (CSG Products normalization)...
Normalized CSG tree has 3 elements
Geometries in cache: 98
Geometry cache size in bytes: 958600
CGAL Polyhedrons in cache: 38
CGAL cache size in bytes: 8037832
Total rendering time: 0:00:02.876
Exporting /tmp/scadfile.scad...
Compiling design (CSG Products normalization)...
Normalized CSG tree has 2 elements
Geometries in cache: 98
Geometry cache size in bytes: 958600
CGAL Polyhedrons in cache: 38
CGAL cache size in bytes: 8037832
Total rendering time: 0:00:00.069

After

Exporting /tmp/scadfile.scad...
ECHO: "render_mount_electronics(1,optics_version='rms');"
WARNING: Viewall and autocenter disabled in favor of $vp*
Compiling design (CSG Products normalization)...
Normalized CSG tree has 3 elements
Geometries in cache: 98
Geometry cache size in bytes: 958600
CGAL Polyhedrons in cache: 38
CGAL cache size in bytes: 8037832
Total rendering time: 0:00:02.880
Exporting /tmp/scadfile.scad...
ECHO: "render_mount_electronics(2,optics_version='rms');"
Compiling design (CSG Products normalization)...
Normalized CSG tree has 2 elements
Geometries in cache: 98
Geometry cache size in bytes: 958600
CGAL Polyhedrons in cache: 38
CGAL cache size in bytes: 8037832
Total rendering time: 0:00:00.070