gg: make draw_rect_empty/5 draw more exact borders, independent of the device, and fitting the draw_rect_filled/5 shapes

[spytheman]

The result of:

module main

import gg
import gx

gg.start(
	bg_color:     gx.white
	window_title: 'Rectangles'
	width:        250
	height:       100
	frame_fn:     fn (ctx &gg.Context) {
		ctx.begin()
		ctx.draw_rect_filled(10, 10, 10, 10, gx.blue)
		ctx.draw_rect_empty(30, 10, 10, 10, gx.red)
		ctx.draw_rect_filled(50, 10, 30, 10, gx.green)
		ctx.draw_rect_empty(100, 10, 30, 10, gx.black)

		ctx.draw_rect_empty(10, 50, 10, 10, gx.blue)
		ctx.draw_rect_filled(30, 50, 10, 10, gx.red)
		ctx.draw_rect_empty(50, 50, 30, 10, gx.green)
		ctx.draw_rect_filled(100, 50, 30, 10, gx.black)
		ctx.end()
	}
)

with this PR on linux (with added guidelines) is:

(the top image is from my GPU, the bottom one is from MESA, rendered with LIBGL_ALWAYS_SOFTWARE=1)

on macos, it is (with added guidelines):

In both cases draw_rect_empty draws a rectangle, that exactly matches the shape and position of a filled rectangle, drawn with draw_rect_filled.

[huly-for-github]

Connected to Huly®: V_0.6-22417