Restore clipping.

[zwcloud]

• [x] Clip every layout group: implemented clipping for layout group at 936ead0
• [x] Clip TextMesh: implemented at d15f2dfacea0de8a79cef1cd6657df0db05cdce0
• [x] Fix: ClientArea clipping rect's range is larger than expected or didn't work for buttons and listboxes inside it. implemented at 50d64808e1b9b7162762e0711f3c8c3840ad8bb7

[zwcloud]

implemented clipping for layout group at https://github.com/zwcloud/ImGui/commit/936ead0118724a258ea6aba31ef15354ac4dde51

detail:

1. Each node will use its parent's ContentRect/Rect as the clip-rect when rendering its Node.content
2. In Node.RenderContent, that clip-rect will be assigned to RenderContext
3. And will be pushed to the clip-rect stack of the GeometryRender in RenderContext.ConsumeContent
4. When the GeometryRender converts content records to mesh, it will assign the top-most clip rect on the stack to DrawCommand.CilpRect
5. DrawCommand.CilpRect will be used by the native renderer: for example, Win32OpenGLRenderer -> glScissor

[zwcloud]

Before: After: