v3.1.0 (2020-05-03)

This minor upgrade adds some fixes and changes that are a bit more than just patches. The text now
has subchapter headings to help readers browse content and get a bit more context. We're introducing
new type aliases point3 and color for vec3 to better indicate the underlying mathematical
types of parameters and variables. Overall, a bunch of small improvements that we'd recommend
adopting, but may warrant comparison with any current projects.

Common

• Fix: Include cmath in vec3.h (#501)
• Fix: Scattered improvements to the text
• New: Subchapters throughout all three books (#267)
• New: Add explanation for padding aarect in the zero dimension (#488)
• Change: Minor change to use new point3 and color type aliases for vec3 (#422)
• Change: Renamed constant_texture to solid_color, add RGB constructor (#452)
• Change: Moved vec3::write_color() method to utility function in color.h header (#502)
• Change: Switch from ffmin/ffmax to standard fmin/fmax (#444, #491)
• Change: Math notation to bold uppercase points, bold lowercase no-barb vectors (#412)
• Change: Books use Markdeep's image class=pixel for rendered image fidelity (#498)

In One Weekend

• Fix: Improve image size and aspect ratio calculation to make size changes easier
• Fix: Added t parameter back into hit_record at correct place
• Fix: image basic vectors off by one
• Fix: Update image and size for first PPM image
• Fix: Update image and size for blue-to-white gradient image
• Fix: Update image and size for simple red sphere render
• Fix: Update image and size for sphere with normal-vector coloring
• Fix: Correct typo in "What's next?" list to rejoin split paragraph on "Lights." Adjust numbering
  in rest of list.
• Change: Define image aspect ratio up front, then image height from that and the image width
• Change: Default image sizes changed from 200x100 to 384x216
• Change: First image size changed to 256x256

The Next Week

• Change: Large rewrite of the image_texture class. Now handles image loading too. (#434)