graphics/simage: disable MPEG2ENC support by default to make packageable

With MPEG2ENC enabled by default, the license is not free, and this port
cannot be packaged.

Therefore the consumers of simage cannot be packaged: among them

Coin-4.0.6 graphics/Coin simage-1.8.4
FreeCAD-1.0.2_7 cad/freecad simage-1.8.4
FreeCAD-devel-r20251228075421_1 cad/freecad-devel simage-1.8.4
py311-pivy-0.6.10 graphics/py-pivy@py311 simage-1.8.4
SoQt-1.6.4,1 x11-toolkits/soqt simage-1.8.4
visp-3.6.0_13 misc/visp simage-1.8.4

This patch make MPEG2ENC optional, so that these ports should be
packageable again.

Problem identified by: Mark Millard

PR: 293070

Approved by: maintainer’s time-out
MFC after: 8 days