Handle precedence of generic and target-specific implementations

[kwokcb]

Fixes #1409

Scenario

It is possible to have a implementation in a derived target which overrides a non-target implementation. The
logic did not try to find implementations with non-empty targets before defaulting to returning an implementation
without a target.

Fix

When dealing with an explicit target on definition search. First test for implementations which have a non-empty target string.
If not found then fallback to non-target implementations. This should fix returning non-target implementations before trying to find implementations with targets.

Test

import MaterialX as mx

# Create a document.
doc = mx.createDocument()

# Create a base and derived targetdef
base_targetDef = doc.addTargetDef("base")
derived_targetDef = doc.addTargetDef("derived")
derived_targetDef.setInheritString('base')

# Create an implementation for the base targetdef
base_impl = doc.addImplementation("IM_function_base")
base_impl.setTarget("base")
base_impl.setNodeDefString("ND_function")

# Create an implementation for the derived targetdef
derived_impl = doc.addImplementation("IM_functon_derived")
derived_impl.setTarget("derived")
derived_impl.setNodeDefString("ND_function")

# Create a nodedef
nodeDef = doc.addNodeDef("ND_function", "float3", "function")

which creates this documet:

<materialx version="1.38">
  <targetdef name="base">
  <targetdef name="derived" inherit="base">
  <implementation name="IM_function_base" target="base" nodedef="ND_function">
  <implementation name="IM_functon_derived" target="derived" nodedef="ND_function">
  <nodedef name="ND_function" node="function">
    <output name="out" type="float3">
</materialx>

The first test returns "derived" and "base", the second before the change returns "" and "". The derived result is incorrect.
After the change, the derived result is "derived" as expected.

# Test with targets specified for base and derived
impl = nodeDef.getImplementation('derived')
print('Got derived target:', impl.getTarget())
impl = nodeDef.getImplementation('base')
print('Got base target: ', impl.getTarget())

# Test with base without a target
base_impl.setTarget('')
impl = nodeDef.getImplementation('derived')
print('Want derived, but got target: "%s"' % impl.getTarget())
impl = nodeDef.getImplementation()
print('Got no target: "%s"' % impl.getTarget())

@dcourtois. Can you see if this will fix your issue? Thanks.

@niklasharrysson, @jstone-lucasfilm : This is the least intrusive change I could think of which does not change
the logic of targetStringsMatch as that method is used in other places.

[dcourtois]

@kwokcb Hi, sorry for the delay in my answer! I finally got to test this, and yes, it fixes my issue and I can finally get rid of my uggly workaround when I use this pull request :)

[kwokcb]

Hi @dcourtois,
No problem -- everyone is busy :).
Good to hear this fixes things. I'll await review on this.

[jstone-lucasfilm]

Looks good to me, thanks @kwokcb!