mesa: ARB_shader_image_load_store not fully required for GLES 3.1
OpenGL ES 3.1 spec includes almost all the functionality of the ARB_shader_image_load_store extension, except the need for formatless storing.
Thus, a driver that fully implements the extension except the formatless storing is still GLES3.1 compliant, but can't expose the extension as supported.
This is a continuation of !13409 (merged)