glsl/linker: Fix out variables linking during single stage

glsl/linker: Fix out variables linking during single stage

Since out variables are copied from shader objects instruction
streams to linked shader instruction steam it should be cloned
at first to keep source instruction steam unaltered.

Fixes: 966a797e433 ("glsl/linker: Link all out vars from a shader
objects on a single stage")

Signed-off-by: Vadym Shovkoplias <vadym.shovkoplias@globallogic.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105731