Add a debug.IsDebug check before splitting strings

[gabstv]

What issue is this addressing?

--

What type of issue is this addressing?

feature

What this PR does | solves

Since debug.IsDebug is a constant, by putting the strings.Cut calls inside the check this makes the (*commandqueue).flush method faster when Ebitengine is compiled without the debug build tag.

Without the if debug.IsDebug check, unnecessary allocations occur (since the logger is a noop logger when debug is disabled):

GC Details after the change:

This happens because Go knows that IsDebug is a constant (and the compiler will optimize this away on production builds).

[hajimehoshi]

LGTM, thanks!