Mesa Adds Option For Changing Intel's OpenGL Driver Default

While originally Intel planned to transition their OpenGL driver default to the modern "Iris" Gallium3D driver rather than the longstanding "i965" DRI driver for Mesa 19.3, that was pushed back to Mesa 20.0 for introduction in Q1'2020. In aiming to make that revised milestone a reality, a new option has been added to Mesa 20.0 with the Meson build system for being able to indicate the Intel OpenGL driver preference...