Fix undefined behavior (not returning a value from a function with return).

Bug: 17379740

This function was missing a return statement, even though it declares a return
type. This is undefined behavior, which clang miscompiled into just falling
through to a later function in some cases. Adding the return statement
fixes the undefined behavior (and probably fixes a bug too).

Change-Id: I05c03b6473b831769dc4fa8b5ba43fb4249f7626

include/ui/mat4.h

@@ -322,6 +322,7 @@ tmat44<T> tmat44<T>::rotate(A radian, const tvec3<B>& about) {
         r[ 1] =  xy*nc + zs;    r[ 5] = y*y*nc +  c;    r[ 9] =  yz*nc - xs;
         r[ 2] =  zx*nc - ys;    r[ 6] =  yz*nc + xs;    r[10] = z*z*nc +  c;
     }
+    return rotation;
 }
 
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