GHOST/wayland: use Wayland only when 'BLENDER_WAYLAND' is set

When the X11 and Wayland backends are available, only query Wayland when
the BLENDER_WAYLAND environment variable is set and notify the user if this
failed.
This makes sure that Blender will always use X11. User can then opt-in to
Wayland for testing and can continue using X11 in case of issues.

Differential Revision: D11489

cmake: disable WITH_GHOST_WAYLAND if any Wayland dependency is missing

This sets WITH_GHOST_WAYLAND to ON and auto-disables it again, if one of
the required Wayland development libraries is missing.

Differential Revision: D11489

GHOST/X11: remove duplicated EGL linking

Differential Revision: https://developer.blender.org/D12034

GHOST/X11: enable EGL

This will replace GLX with EGL for X11. GLEW does not support GLX and EGL
at the same time. Most distributions build GLEW with GLX support, so we
have to use the externally provided GLEW and build with EGL support.
This effectively sets WITH_SYSTEM_GLEW to OFF for all Linux configurations.

Differential Revision: https://developer.blender.org/D12034

GHOST/X11: link libGL for X11-EGL to resolve OpenGL symbols

This links libGL if X11-EGL is enabled, since libOpenGL is not installed by
default on some distributions. This is a workaround to resolve the OpenGL
symbols that would otherwise be resolved via GLVND.

Differential Revision: https://developer.blender.org/D12034

GHOST/X11: don't link GLX when using EGL

Differential Revision: https://developer.blender.org/D12034