Added POV image effect

Setup is really easy, after first boot and WiFi/LEDs setup:
go to wled.local/edit and upload a couple image to WLed's filesystem.
Only PNG is supported right now, further support for GIF is planned.
The image should be as wide as the 1D segment you want to apply to.

When done, go to the Effect page on the UI, select "POV Image" effect.

You could also update the image with a post to the JSON-API like this:
curl -X POST http://[wled]/json/state -d '{"seg":{"id":0,"fx":114,"f":"/axel.png"}}'

The segment should move at around 120RPM (that's 2revolutions per seconds) for an image to showup.
More informations and pictures here : https://lumina.toys

wled00/const.h

@@ -200,6 +200,7 @@
 #define USERMOD_ID_INA226                50     //Usermod "usermod_ina226.h"
 #define USERMOD_ID_AHT10                 51     //Usermod "usermod_aht10.h"
 #define USERMOD_ID_LD2410                52     //Usermod "usermod_ld2410.h"
+#define USERMOD_ID_POV_DISPLAY           53     //Usermod "usermod_pov_display.h"
 
 //Access point behavior
 #define AP_BEHAVIOR_BOOT_NO_CONN          0     //Open AP when no connection after boot