Build: Fix install rule for resources install into build folder.

[kwokcb]

Issue

The post build step for MaterialXTest does no trigger so resources are not copied to the
build area on a build.

This means developers must manually copy this over when they are changing any files affecting the resources folder. If the lack of copy is not noticed / done then it can get confusing as to why source changes are not reflected in the build tests.

Change

Add an explicit target for resource copy and make MaterialXTest dependeng on it.

[jstone-lucasfilm]

This looks very promising, thanks @kwokcb, and I'll just add a note that we should time our build process before and after this change, to make sure we're maintaining build performance.

[kwokcb]

It doesn't have any affect on the initial build since resources are always installed so CI build time won't change.

To check however, I put a timer in the build step and get this so it's pretty negligable.

Copying OSL utilities to build directory...
Elapsed time (seconds): 0.00699933
[ 68%] Built target MaterialXTestOslUtilities
[ 68%] Copying resources to build directory
Copying resources to build directory...
Elapsed time (seconds): 0.0909475

I could check in the timer code but don't think it's needed.

[kwokcb]

After chatting with @lkerley on Slack, I've made the small change to only copy if files have changed to minimize the time spent in case the file system is slower.

[kwokcb]

Copy only files which have changed.

[kwokcb]

I've done some timings on my machine which is a 4 yr old Alienware personal laptop.

Clean builds show about 2 seconds for resources. libraries which we already copy is about a second.
Without this change this is already the overhead when building with clean resources and libraries.

Incremental builds are about 1 second.

// Clean build
[MaterialXTestData] Copying data libraries...
  Elapsed time (seconds): 0.944269
  Copying OSL utilities to build directory
  [MaterialXTestOslUtilities] Copying OSL utilities...
  Elapsed time (seconds): 0.0731214
  Copying resources to build directory
  [MaterialXTestResources] Copying resources...
  Elapsed time (seconds): 1.86759

   [MaterialXTestData] Copying data libraries...
  Elapsed time (seconds): 0.951778
  Copying OSL utilities to build directory
  [MaterialXTestOslUtilities] Copying OSL utilities...
  Elapsed time (seconds): 0.0723189
  Copying resources to build directory
  [MaterialXTestResources] Copying resources...
  Elapsed time (seconds): 1.87808 

// Incremental build
  [MaterialXTestData] Copying data libraries...
  Elapsed time (seconds): 0.206294
  Copying OSL utilities to build directory
  [MaterialXTestOslUtilities] Copying OSL utilities...
  Elapsed time (seconds): 0.0529004
  Copying resources to build directory
  [MaterialXTestResources] Copying resources...
  Elapsed time (seconds): 0.728033

  [MaterialXTestData] Copying data libraries...
  Elapsed time (seconds): 0.194733
  Copying OSL utilities to build directory
  [MaterialXTestOslUtilities] Copying OSL utilities...
  Elapsed time (seconds): 0.0547186
  Copying resources to build directory
  [MaterialXTestResources] Copying resources...
  Elapsed time (seconds): 0.705728