function getShapeClass(name) {
  if (_customShapeMap.hasOwnProperty(name)) {
    return _customShapeMap[name];
  }
}
Find shape class registered by `registerShape`. Usually used in
fetching user defined shape.

[Caution]:
(1) This method **MUST NOT be used inside echarts !!!**, unless it is prepared
to use user registered shapes.
Because the built-in shape (see `getBuiltInShape`) will be registered by
`registerShape` by default. That enables users to get both built-in
shapes as well as the shapes belonging to themsleves. But users can overwrite
the built-in shapes by using names like 'circle', 'rect' via calling
`registerShape`. So the echarts inner featrues should not fetch shapes from here
in case that it is overwritten by users, except that some features, like
`custom series`, `graphic component`, do it deliberately.

(2) In the features like `custom series`, `graphic component`, the user input
`{tpye: 'xxx'}` does not only specify shapes but also specify other graphic
elements like `'group'`, `'text'`, `'image'` or event `'path'`. Those names
are reserved names, that is, if some user register a shape named `'image'`,
the shape will not be used. If we intending to add some more reserved names
in feature, that might bring break changes (disable some existing user shape
names). But that case probably rearly happen. So we dont make more mechanism
to resolve this issue here.

@param name
@return The shape class. If not found, return nothing.