Make attached meshes use their own blit parameters (#1095)

This might break some scenarios which for example change the color modulation
of a clonk and then expect that also its shovel or other objects get colored.
Such scenarios need to be fixed by also coloring the attached objects. This
might not be very convenient, and maybe we should introduce an option for
attached meshes to use the old behaviour (take blit parameters from main
object).

However, let's see how this change turns out in practise and then we see
whether further modifications are needed.
Allow distributions to use the system tinyxml.

Add a new cmake option WITH_SYSTEM_TINYXML=ON that allows to use the system
tinyxml library and header instead of the bundled copy. Adjust the include
path in the single source file referencing the header and add the thirdparty
path to the include search path if the use system option is unset or off.