Update `.gitignore` to deal with render-test output
Update `.gitignore` to deal with render-test output

The rule of thumb so far is that expected output for `.slang` files should be checked in, but not for `.hlsl` or `.glsl`. The render test breaks that rule, because we never want to check in the expected output.

csyonghe/slang,csyonghe/slang,shader-slang/slang,shader-slang/slang,shader-slang/slang,csyonghe/slang