[web] Implement path winding. Fix path bounds for multirect.

[ferhatb]

Fixes cupertino alert rendering.
Related issues:
flutter/flutter#49877
flutter/flutter#51701

1. drawPath now passes winding rule to html canvas.
2. Fixed Path.getBounds and added test.
3. Added code to skip empty paths in recording canvas.
4. Optimized runPath code. dart2js change below:

Before:

_runPath$2: function(ctx, path) {
      var t1, t2, _i, t3, t4, _i0, command;
      ctx.beginPath();
      for (t1 = path.subpaths, t2 = t1.length, _i = 0; _i < t1.length; t1.length === t2 || (0, H.throwConcurrentModificationError)(t1), ++_i)
        for (t3 = t1[_i].commands, t4 = t3.length, _i0 = 0; _i0 < t3.length; t3.length === t4 || (0, H.throwConcurrentModificationError)(t3), ++_i0) {
          command = t3[_i0];
          switch (command.type) {
            case 5:....

After:

_runPath$2: function(ctx, path) {
	      var subpaths, subpathCount, subPathIndex, commands, commandCount, c, command;
	      ctx.beginPath();
	      subpaths = path.subpaths;
	      subpathCount = subpaths.length;
	      for (subPathIndex = 0; subPathIndex < subpathCount; ++subPathIndex) {
	        commands = subpaths[subPathIndex].commands;
	        commandCount = commands.length;
	        for (c = 0; c < commandCount; ++c) {
	          command = commands[c];
	          switch (command.type) {
	            case 5:

Winding rule test:

[yjbanov]

Nice! I think I saw this in the cupertino slider as well. There were some commands thrown into the canvas without anything actually painted.