fix implicit 64 to 32 bit conversion warnings

[antond-weta]

Description

Enabling the implicit 64 to 32 bit conversion warnings shows several hundreds of them. This is my attempt to fix them all.

Tests

I have built the code on an Apple silicon Mac (both command line and Xcode), and an x64 linux machine, and have run the tests. I have not tested on Windows, or any 32-bit hardware.

I'm not familiar with the project enough to say that my testing was adequate. Any help would be appreciated.

Checklist:

• [x] I have read the contribution guidelines.
• [x] I have updated the documentation, if applicable.
• [ ] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• [ ] If I added or modified a C++ API call, I have also amended the corresponding Python bindings (and if altering ImageBufAlgo functions, also exposed the new functionality as oiiotool options).
• [x] My code follows the prevailing code style of this project. If I haven't already run clang-format before submitting, I definitely will look at the CI test that runs clang-format and fix anything that it highlights as being nonconforming.