introduce mp4flag to make file read/write mode optional

GPDeps

plus these additional changes from the gp8 branch
- HAL-12861: fixing warnings
- reference the pkgconfig file explicitly in the CMAKE_CURRENT_BINARY_DIR
  to support builds that are not launched from the binary build dir
- Drop down the minimum CMake version to 3.1

HAL-19804. Make logic more robust

GPDeps

plus these additional changes from the gp8 branch
- HAL-12861: fixing warnings
- reference the pkgconfig file explicitly in the CMAKE_CURRENT_BINARY_DIR
  to support builds that are not launched from the binary build dir
- Drop down the minimum CMake version to 3.1

Fix for reading GPMF payloads form the udta atom within MP4s.

fix for mp4 with only one GPMF payload

version 2.0 - security and memory fixes

version 1.5.1 includes minor changes preparing for v2

Update README.md

Add new camera metadata descriptions

GPDeps

plus these additional changes from the gp8 branch
- HAL-12861: fixing warnings
- reference the pkgconfig file explicitly in the CMAKE_CURRENT_BINARY_DIR
  to support builds that are not launched from the binary build dir
- Drop down the minimum CMake version to 3.1