Faster dashed line generation (#1027)

* Faster dashed lines generation

* Add dashed_line benchmark

epaint/Cargo.toml

@@ -63,3 +63,10 @@ single_threaded = ["atomic_refcell"]
 # Only needed if you plan to use the same fonts from multiple threads.
 # It comes with a minor performance impact.
 multi_threaded = ["parking_lot"]
+
+[dev-dependencies]
+criterion = { version = "0.3", default-features = false }
+
+[[bench]]
+name = "benchmark"
+harness = false