fix POV Display usermod (wled#4427)

* POV Display usermod

this usermod adds a new effect called "POV Image".
To get it to work:
- read the README :)
- upload a bmp image to the ESP filesystem using "/edit" url.
- select "POV Image" effect.
- set the filename (ie: "/myimage.bmp") as segment name.
- rotate the segment at approximately 20 RPM.
- enjoy the show!
* improve file extension checks
* improve README, remove PNGdec reference, clean usermod
* restrain to esp32 platform + reduce memory footprint with malloc

Merge pull request wled#4028 from Aircoookie/dependabot/pip/urllib3-1…

….26.19

Bump urllib3 from 1.26.18 to 1.26.19

Update platformio.ini

Update platformio.ini