In GLSL ES functions can have precision qualifers for the return type.
Interpreting this quote from the GLSL ES spec, it seems like the
precision for the return type should match between repeated
declarations of the same function:

“Only a precision qualifier is allowed on the return type of a
 function. […] If a function name is declared twice with the same
 parameter types, then the return types and all qualifiers must also
 match, and it is the same function being declared.”

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>