glsl: Fix for scalar float built-in definitions.

These need abs, and we need more tests.

src/glsl/builtin_function.cpp

@@ -724,7 +724,7 @@ static const char *builtin_distance =
    "     (parameters\n"
    "       (declare (in) float p0)\n"
    "       (declare (in) float p1))\n"
-   "     ((return (expression float - (var_ref p0) (var_ref p1)))))\n"
+   "     ((return (expression float abs (expression float - (var_ref p0) (var_ref p1))))))\n"
    "\n"
    "   (signature float\n"
    "     (parameters\n"
@@ -1302,7 +1302,7 @@ static const char *builtin_length =
    "   (signature float\n"
    "     (parameters\n"
    "       (declare (in) float arg0))\n"
-   "     ((return (var_ref arg0))))\n"
+   "     ((return (expression float abs (var_ref arg0)))))\n"
    "\n"
    "   (signature float\n"
    "     (parameters\n"

src/glsl/builtins/ir/distance

@@ -3,7 +3,7 @@
      (parameters
        (declare (in) float p0)
        (declare (in) float p1))
-     ((return (expression float - (var_ref p0) (var_ref p1)))))
+     ((return (expression float abs (expression float - (var_ref p0) (var_ref p1))))))
 
    (signature float
      (parameters

src/glsl/builtins/ir/length

@@ -2,7 +2,7 @@
    (signature float
      (parameters
        (declare (in) float arg0))
-     ((return (var_ref arg0))))
+     ((return (expression float abs (var_ref arg0)))))
 
    (signature float
      (parameters