v3.1.1 (2020-05-16)

Common

• Fix: Refactoring the camera code in v3.1.0 missed updating the viewport to match, resulting in
  distorted renders (#536)
• Change: Camera code improvements to make it more robust when any particular value changes. Also,
  the code develops in a smoother series of iterations as the book progresses. (#536)

In One Weekend

• Fix: Camera initialization with explicit up vector (#537)
• Fix: Changed some text around the camera model and the camera defocus blur model (#536)
• Change: The C++ <random> version of random_double() no longer depends on <functional>
  header.
• Change: Refactored random_scene(). More named intermediate values, sync'ed with source.
  (#489)

The Next Week

• Fix: Added clarification about updating lambertian variables from color to solid_color.
• Fix: Corrected for-loop indices (they differed from the version in book 1) in random_scene().
• Fix: Introduce "Texture Coordinates for Spheres" in Chapter 4 to support (u,v) coordinates in
  hit_record (#496)
• Fix: Small correction: we now use std::sort instead of qsort (#490)
• Change: Refactored random_scene(). More named intermediate values, sync'ed with version in
  In One Weekend and with source. Added highlight for update from last version in book 1. (#489)
• Change: The C++ <random> version of random_double() no longer depends on <functional>
  header