Remove compound assignment restrictions from WEBGL_debug_shader_precision

It is possible to fully emulate precision for compound assignment by
replacing compound assignments with calls to the following kind of a
function:

float webgl_compound_add(inout float x, in float y) {
    x = round_float(x + y);
    return x;
}
Merge pull request #785 from NVIDIA/remove-precision-emulation-limitations

Remove compound assignment restrictions from WEBGL_debug_shader_precision