[SofaMatrix.imgui] Custom style for the rendered matrix

[alxbilger]

Additional features when visualizing a matrix in the GUI:

• Custom colors: By default, it takes the colors ImGuiCol_Text for non-zero values and ImGuiCol_WindowBg for zero values. The image is more seamlessly integrated into the GUI.
• Color picking: colors can be changed by the user (for now they are not saved persistently).
• Save button: A button save the image along the executable file.

matrix.mp4

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• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).