[GUI] Support images with 2 channels (RG) in GUI.set_image

[archibate]

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This could be handy when visualizing 2D vector fields, e.g.: velocity field in stable_fluid.py.

[codecov]

Codecov Report

Merging #1624 into master will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1624      +/-   ##
==========================================
+ Coverage   45.06%   45.07%   +0.01%     
==========================================
  Files          40       41       +1     
  Lines        5790     5766      -24     
  Branches     1012     1053      +41     
==========================================
- Hits         2609     2599      -10     
+ Misses       3032     3009      -23     
- Partials      149      158       +9     

Impacted Files	Coverage Δ
python/taichi/lang/meta.py	62.31% <0.00%> (-2.39%)	⬇️
python/taichi/misc/gui.py	0.00% <0.00%> (ø)
python/taichi/lang/ast_checker.py	70.58% <0.00%> (-1.64%)	⬇️
python/taichi/lang/linalg.py	89.33% <0.00%> (-0.67%)	⬇️
python/taichi/lang/__init__.py	54.30% <0.00%> (-0.45%)	⬇️
python/taichi/misc/util.py	17.00% <0.00%> (-0.33%)	⬇️
python/taichi/main.py	23.99% <0.00%> (-0.14%)	⬇️
python/taichi/core/util.py	0.25% <0.00%> (-0.05%)	⬇️
python/taichi/testing.py	76.92% <0.00%> (ø)
... and 12 more

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[Rullec]

LGTM!
Only curious: Is it a good way to define a literally "2D" image by only truncate the red and green channel? I'm curious about its performance.

[archibate]

LGTM!
Only curious: Is it a good way to define a literally "2D" image by only truncate the red and green channel? I'm curious about its performance.

In fact we are always using RGBA as internal format storing in our GUI module, no matter grayscale, RG or RGB.

[zhai-xiao]

Great job! Thanks, @archibate

[zhai-xiao]

Looks great! Thanks 🧀