sokol_gfx.h d3d11: add sg_desc.d3d11_shader_debugging bool flag

[floooh]

Fixes: #1043

Basically call sg_setup() like this:

    sg_setup(&(sg_desc){
        .environment = sglue_environment(),
        .logger.func = slog_func,
        .d3d11_shader_debugging = true,
    });

...and then just make sure to feed HLSL source code into sg_make_shader() instead of bytecode (e.g. when using sokol-shdc, omit the -b / --bytecode cmdline option.