Implement proper copy constructor for ResourceManager to allow mesh copying.

Fixes #5.
9 jobs for mesh_copying in 3 minutes and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #40217
VS2013
VS2013-32-bit

00:00:50

passed #40216
VS2013
VS2013-64-bit

00:01:01

passed #40215
VS2015
VS2015-32-bit

00:01:01

passed #40214
VS2015
VS2015-64-bit

00:01:10

passed #40211
Linux
clang-c++11

00:03:48

passed #40213
Linux
cppcheck

00:00:29

passed #40218
Linux
doc

00:00:20

passed #40210
Linux
gcc-c++11

00:03:59

passed #40212
Apple
macos-c++11

00:01:27