glsl/linker: Don’t check precision for shader interface

glsl/linker: Don’t check precision for shader interface

On GLES, the interface between vertex and fragment shaders doesn’t
need to have matching precision.

Section 4.3.10 of the GLSL ES 3.00 spec:

“The type of vertex outputs and fragment inputs with the same name
 must match, otherwise the link command will fail. The precision does
 not need to match.”

Reviewed-by: Eric Anholt <eric@anholt.net>