Blender Git Loki

Fix T80141: Fix Compiling Workbench Volume Shaders (Mesa 20.0.8)

Default mesa driver for ubuntu 20.04 fails when a name is defined twice.
M_PI is defined in both `common_workbench_lib` and `common_math_lib`. This patch
remove the define out of common_workbench_lib

For reference it fails on https://github.com/mesa3d/mesa/blob/mesa-20.0.8/src/compiler/glsl/glcpp/glcpp-parse.y#L1186
during the check if the macros are the same.

Reviewed By: Cl�ment Foucault

Differential Revision: https://developer.blender.org/D8741