shader: Also don't convert when src type is SIGNED and input is int.
Closes: #18 (closed)
Fixes: dEQP- GLES31.functional.geometry_shading.layered.fragment_layer_2d_array dEQP- GLES31.functional.geometry_shading.layered.fragment_layer_2d_multisample_array dEQP-GLES31.functional.geometry_shading.layered.fragment_layer_3d dEQP-GLES31.functional.geometry_shading.layered.fragment_layer_cubemap