Invalid DIB size calculation

[ForNeVeR]

DeviceIndependentBitmap currently doesn't take into account the color palette stored in the DIB, which makes the size calculation incorrect. Currently it reads:

• read 40-byte header
• read (seemingly properly calculated amount of) data bytes into Data

But it should:

• read 40-byte header
• read (header.ColorsUsed ?? 1 << header.BitCount) * 4 bytes into Colors
• read same amount of data into Data

Here's a compressed example of a file that reads incorrectly by the library: _bm0.zip.

[ForNeVeR]

I've sent a PR with a fix to the fork I use: wieslawsoltes#1. Still not sure what's the story with the fork and NuGet, though. I'm ready to send a PR to this repo as well if you want.