[sans] Fix italic U+2098-2099 (subscript m & n)

They are now references of m.inferior and n.inferior, respectively. This
fixes #72.

They will now look serif though, but so are other *script characters in
this font.

sources/libertinussans-italic.sfd

@@ -68043,62 +68043,22 @@ EndChar
 
 StartChar: uni2098
 Encoding: 8344 8344 2191
-Width: 388
-VWidth: 960
-Flags: MW
+Width: 485
+VWidth: 442
+Flags: HMW
 LayerCount: 2
 Fore
-SplineSet
-169.9 531 m 0
- 153.7 455 201.5 393 277.5 393 c 0
- 354.5 393 428.7 455 444.9 531 c 0
- 461.1 607 413 668 336 668 c 0
- 260 668 186.1 607 169.9 531 c 0
-140.9 531 m 0
- 160.2 622 251.2 697 342.2 697 c 0
- 434.2 697 492.2 622 472.9 531 c 0
- 453.3 439 363.6 365 271.6 365 c 0
- 180.6 365 121.3 439 140.9 531 c 0
-326.3 547 m 1
- 412.3 547 l 1
- 405.7 516 l 1
- 319.7 516 l 1
- 301.4 430 l 1
- 270.4 430 l 1
- 288.7 516 l 1
- 201.7 516 l 1
- 208.3 547 l 1
- 295.3 547 l 1
- 313.8 634 l 1
- 344.8 634 l 1
- 326.3 547 l 1
-EndSplineSet
+Refer: 2318 -1 N 1 0 0 1 0 0 3
 EndChar
 
 StartChar: uni2099
 Encoding: 8345 8345 2192
-Width: 388
-VWidth: 960
-Flags: MW
+Width: 329
+VWidth: 442
+Flags: HMW
 LayerCount: 2
 Fore
-SplineSet
-169.9 531 m 0
- 153.7 455 201.5 393 277.5 393 c 0
- 354.5 393 428.7 455 444.9 531 c 0
- 461.1 607 413 668 336 668 c 0
- 260 668 186.1 607 169.9 531 c 0
-140.9 531 m 0
- 160.2 622 251.2 697 342.2 697 c 0
- 434.2 697 492.2 622 472.9 531 c 0
- 453.3 439 363.6 365 271.6 365 c 0
- 180.6 365 121.3 439 140.9 531 c 0
-405.7 516 m 1
- 201.7 516 l 1
- 208.3 547 l 1
- 412.3 547 l 1
- 405.7 516 l 1
-EndSplineSet
+Refer: 2319 -1 N 1 0 0 1 0 0 3
 EndChar
 
 StartChar: uni1FBD