qtcoin uses ReadTrimesh to load render meshes

QtCoin previously only tried to load render geometry (i.e. the mesh returned by
GetRenderFilename) with an internal loader. This loader does not support
several types of meshes (notably COLLADA and, in some cases, STL) that are
supported for collision geometry. Collision geometry is loaded through a
different pipeline, which calls ReadTrimesh.

I changed the logic in QtCoin to fall back on ReadTrimesh if loading the render
geometry fails. This replaces the previous fall-through logic which would
immediately fall back on displaying the collision mesh. This is very important
for visual-only geometry that does have collision geometry (e.g. using GT_None
or by specifying a trivial primitive).