Lazily import matplotlib

[twoertwein]

Matplotlib is a large and slow to import dependecy of mediapipe but it is used in only one function. Currently matplotlib is imported when calling import mediapipe.

This MR moves the matplotlib import inside the only function that depends on matplotlib. For people who don't call this function:

• the import time will be faster
• they could uninstall matplotlib and still use mediapipe (related to Make it possible to let mediapipe be installed in headless environments #3661)

(Ideally, matplotlib would become an optional dependecy.)

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.