Changelog: add missing updates since v4 alpha.1

Author: hollasch

CHANGELOG.md

@@ -8,13 +8,22 @@ then.
 
 ### Common
   - Fix - Fix references from `random_in_hemisphere()` to `random_on_hemisphere()` (#1198)
+  - Change - We've removed the few cases where we used C++ default constructors. Instead, we either
+             require all parameters, or use operator overloading to use default values.
+  - Change - Increase compile warning levels for MSVC.
+  - Change - For clarity across audiences with broad programming backgrounds, we now use `double(x)`
+             instead of `static_cast<double>(x)`, and similarly for other types.
 
 ### In One Weekend
   - Change - Update reference to "Fundamentals of Interactive Computer Graphics" to "Computer
     Graphics: Principles and Practice". This is the name used by newer editions of the book.
+  - Change - New BVH optimization splits the bounds according to the longest bounding box dimension,
+             yielding a 15-20% speedup (#1007)
 
 ### The Next Week
   - Change - `perlin::turb()` no longer defaults the value for the depth parameter.
+  - New - add section on alternative 2D primitives such as triangle, ellipse and annulus (#1204,
+          #1205)
 
 ### The Rest of Your Life