Add borders to BitMap in BitMapEditor

[arkology]

Problem: try to guess where are borders of BitMap when using Black OLED theme:

I guess same problem might be if someone is using full white theme.

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Solution: Draw outline around BitMap (TextureRect) - same way as #100569.

Preview for different themes:

Default theme	Light	Black OLED	passivestar minimal theme

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Note: There is a change in behavior of BitMapEditor. Previously TextureRect used STRETCH_KEEP_ASPECT_CENTERED so big textures tried to occupy all space they need:

Now BitMap texture will occupy up to 250 in height and proportional value in width.

For small-sized BitMap nothing has changed. Video below shows "after/before". "After" with outline around texture, "before" - without.

godot.windows.editor.dev.x86_64_xghhaWszCO.mp4