gl_shader_manager: Take ShaderSetup instances by const reference in...

gl_shader_manager: Take ShaderSetup instances by const reference in UseProgrammableVertexShader() and UseProgrammableFragmentShader()

Avoids performing unnecessary copies of 65560 byte sized ShaderSetup
instances, considering it's only used as part of lookup and not
modified.

Given the parameters were already const, it's likely taking these
parameters by reference was intended but the ampersand was forgotten.