During package build it failed reporting conflict w/ /usr/lib64/libEGL.so.1 (plus some pkgconfig files). New mesa (19.1.3-r3) installed /usr/lib64/libEGL.so.1.1.0. These conflicting .so and .so.1 files were removed and mesa re-installed successfully.

However, it appears that link references to libraries were not restored by package. This resulted in build failure in dependent packages. During build of package x11-libs/vte, it failed reporting missing library reference:

/usr/lib/gcc/x86_64-pc-linux-gnu/9.1.1/../../../../x86_64-pc-linux-gnu/bin/ld: warning: libEGL.so.1, needed by /usr/lib/gcc/x86_64-pc-linux-gnu/9.1.1/../../../../lib64/libcairo.so, not found (try using -rpath or -rpath-link)

Manually adding symbolic links /usr/lib64/libEGL.so and /usr/lib64/libEGL.so.1 to /usr/lib64/libEGL.so.1.1.0 allowed build to resume without error.