fix28 overflow

[rs1729]

VGA_Graphics/Mandelbrot_Set/mandelbrot_fixvfloat.c

Hunter-Adams-RP2040-Demos/VGA_Graphics/Mandelbrot_Set/mandelbrot_fixvfloat.c

Line 188 in f5f85ba

if ((Zre_sq + Zim_sq) >= FOURfix28) break ;

The fix28 values Zre_sq + Zim_sq can overflow, resulting in artifacts.
You can mitigate the problem a little by

    if ((unsigned)(Zre_sq + Zim_sq) > (unsigned)FOURfix28) break;

(or using fix27, but losing precision).
However some points remain for which the iterates will overflow.
Probably better to check for potential overflow before the escape check (affects only iterates outside the circle)

    if (Zre < -TWOfix28 || Zre > TWOfix28) break;
    if (Zim < -TWOfix28 || Zim > TWOfix28) break;

    if ((unsigned)(Zre_sq + Zim_sq) > (unsigned)FOURfix28) break;

In principle the rectangle check is an escape check, so it would also work (the rectangle contains the circle), but the contour lines outside the Mandelbrot set would look different.