Dot files filtering

[ondrejhanak]

Hi all, I saw there was a code to filter SD card content and process only BMP files (most importantly skipping dot files from macOS). Introduced as part of #2 by @bmuessig, but later removed by larger commit aaa5498 from @shhds . Was this intentional guys? Looks useful to me.

[bmuessig]

I feel like they initially merged everything then back-tracked. My firmware has been running fine on my frame for months now. The only code that changes things to be different from their instructions is the gamma correction code, as it will affect bitmaps manually processed. I have kept that code in a separate PR for that reason.