Add box_shadow spread, h_offset, and v_offset

[maun]

Hi,

I added spread, h_offset, and v_offset to the box shadows. I tried to imitate the CSS property definition. inset is missing, as no clipping is done. Also only one box shadow style is supported, because I could not think of a good style API for multiple. If you got a good idea how to set multiple box shadows I can implement it. If you want me to, I could also try to add the inset.

Some screenshots:

Offsets

stack((empty().style(|s| {
    s.size_px(200.0, 100.0)
        .background(Color::CYAN)
        .box_shadow_h_offset(30.0)
        .box_shadow_v_offset(20.0)
}),))
.style(|s| s.size_pct(100.0, 100.0).justify_center().items_center())

stack((empty().style(|s| {
    s.size_px(200.0, 100.0)
        .background(Color::CYAN)
        .box_shadow_h_offset(30.0)
        .box_shadow_v_offset(20.0)
        .border_radius(16.0)
}),))
.style(|s| s.size_pct(100.0, 100.0).justify_center().items_center())

Spread

stack((empty().style(|s| {
    s.size_px(200.0, 100.0)
        .background(Color::CYAN)
        .box_shadow_spread(20.0)
}),))
.style(|s| s.size_pct(100.0, 100.0).justify_center().items_center())

stack((empty().style(|s| {
    s.size_px(200.0, 100.0)
        .background(Color::CYAN)
        .box_shadow_spread(20.0)
        .border_radius(16.0)
}),))
.style(|s| s.size_pct(100.0, 100.0).justify_center().items_center())

Combined with blur

stack((empty().style(|s| {
    s.size_px(200.0, 100.0)
        .background(Color::CYAN)
        .box_shadow_spread(2.0)
        .box_shadow_h_offset(10.0)
        .box_shadow_v_offset(10.0)
        .box_shadow_blur(8.0)
        .box_shadow_color(Color::rgb(0.5, 0.5, 0.5))
        .border_radius(16.0)
}),))
.style(|s| s.size_pct(100.0, 100.0).justify_center().items_center())

[codecov]

Codecov Report

Merging #108 (07ea953) into main (1b62ae9) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##            main    #108      +/-   ##
========================================
- Coverage   3.74%   3.72%   -0.03%     
========================================
  Files         51      51              
  Lines       8564    8620      +56     
========================================
  Hits         321     321              
- Misses      8243    8299      +56     

Files Changed	Coverage Δ
src/style.rs	22.54% <0.00%> (-1.77%)	⬇️
src/view.rs	0.00% <0.00%> (ø)

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[dzhou121]

Excellent work! Thanks.