This fixes:

1. A compilation error when assigning scalars to matricies

2. A semantic error in matrix construction from scalars.  This was
initializing the diagonal, where HLSL semantics require the scalar be
replicated to every matrix element.

3. Functions accepting mats can be called with scalars, which will
be shape-converted to the matrix type.  This was previously failing
to match the function signature.

NOTE: this does not yet handle complex scalars (a function call,
say) used to construct matricies.  That'll be added when the
node replicator service is available.  For now, there's an assert.

There's one new test (hlsl.scalar2matrix.frag).  An existing test
lsl.type.half.frag changes, because of (2) above, and a negative
test error message changes due to (3) above.

Fixes #923.