Add a test for type combinations allowed for compound assignment

There are bugs where browsers and/or drivers allow compound assignments
to an lvalue of a scalar type with an rvalue of a vector/matrix type,
even though they should not. Add a test that covers this plus other
combinations for compound assignment. It is not an error when the lvalue
has a vector/matrix type and the rvalue has a scalar type.

This is the first test for the 1.0.4 test suite, so also update test
suite version number to 1.0.4 in code and documentation.
Should the messages refer to the "sub" operation too? Otherwise it might look like there are duplicated tests in the output.