fix issue with negation of (bool->int) result

[fitzymj]

An OPCODE_NOT followed by a SVT_BOOL->SVT_INT conversion can result in the following when the needsBoolUpscale block is hit:

bool myBool = false;
int result = ~(myBool) * int(0xffffffffu);

This PR emits the correct GLSL, wrapping the upscale operation in parentheses:

int result = ~((myBool) * int(0xffffffffu));

It applies the same fix to generated Metal Shading Language.

[aet]

Thanks! I've applied the change manually into the latest batch of changes from upstream, due to indentation changes etc