glcpp: Skip unnecessary line continuations removal

glcpp: Skip unnecessary line continuations removal

Overwhelming majority of shaders don't use line continuations. In my
shader-db only shaders from the Talos Principle and Serious Sam used
them, less than 1% out of all shaders. Optimize for this case, don't
do any copying if no line continuation was found.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>