Scaling issue in the Vulkan/GLFW demo on retina display

[csheinlen]

I had scaling issues with the Vulkan/GLFW demo causing the UI to render at half size in the upper left quadrant of the window in macOS. I was able fix this by using the scale factor in glfw.fb_scale when setting the width/height values in render_pass_begin_info and viewport in nk_glfw3_render:

render_pass_begin_nfo.renderArea.extent.width = (uint32_t)(glfw.width * glfw.fb_scale.x);
render_pass_begin_nfo.renderArea.extent.height = (uint32_t)(glfw.height * glfw.fb_scale.y);
...
viewport.width = (float)(glfw.width * glfw.fb_scale.x);
viewport.height = (float)(glfw.height * glfw.fb_scale.y);

[RobLoach]

Would you be able to send a pull request?