ShaderExecutor: read counter values from the buffer
Problem: At vktOpaqueTypeIndexingTests.cpp:1077, the tests creates a std::vector called counterValues and default-initializies. Then, in the loop below, it checks counterValues as if it contains the actual counter values even though they are never read from the buffer.
Solve: Read back the counter values from the buffer.
Fixes #175
See merge request !291