fix POV Display usermod (#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

usermods/pov_display/library.json

@@ -0,0 +1,5 @@
+{
+  "name:": "pov_display",
+  "build": { "libArchive": false},
+  "platforms": ["espressif32"]
+}