Fix broken tests:

* Remove "readonly" keyword from SSBO declarations in shaders. The "readonly" keyword is part of GLSL 420 and GL_ARB_image_load_store. When the shader version was dropped from #version 420 to #version 410 for the attempted Mac port, this keyword became unsupported. GL_ARB_shader_storage_buffer_object allows the "readonly" keyword (along with several others) to be used on SSBOs, but it is written against GLSL 420 and does not explicitly re-introduce these keywords on earlier versions of GLSL. A sufficiently smart shader compiler should see that the buffer is only ever read from and optimize accordingly anyway.

* In the bindless tests, do not pass NVIDIA-specific enums to EnableClientState or DisableClientState during shutdown when the corresponding extension is not present. Prior to this change, should the test fail to initialize due to the extension being missing, shutdown would still be executed and this would leave errors dangling.