Fixed setting the file size when the BMP encoder has a color palette

[dlemstra]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

The colorPaletteSize is also part of the file size.

Fixes #1380.

[codecov]

Codecov Report

Merging #1384 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1384   +/-   ##
=======================================
  Coverage   82.85%   82.85%           
=======================================
  Files         690      690           
  Lines       31074    31074           
  Branches     3512     3512           
=======================================
  Hits        25747    25747           
  Misses       4605     4605           
  Partials      722      722           

Flag	Coverage Δ
#unittests	82.85% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs	96.66% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 581efcf...7a8615b. Read the comment docs.