feat: legend scrolling

[brenocq]

Closes #193

This pull request adds legend scrolling functionality to the 3D plot legend in ImPlot3D, improves legend rendering and interaction, and updates the demo to showcase the new feature. The main changes include implementing scrollable legends, updating legend state and rendering logic, and enhancing the demo for better usability.

Legend scrolling and interaction:

• Added a ClampLegendRect function to restrict the legend rectangle within the plot bounds, and updated legend rendering to support scrolling when the legend content exceeds available space. Mouse wheel scrolling is enabled when hovering over the legend. (implot3d.cpp) [1] [2]
• Updated the legend's hover and interaction logic to use the clamped rectangle, preventing off-screen overflow from registering as hovered. (implot3d.cpp)

Legend state and structure updates:

• Added Scroll and RectClamped members to ImPlot3DLegend, and an ID member to ImPlot3DItemGroup, initializing and updating these as needed. (implot3d_internal.h, implot3d.cpp) [1] [2] [3]
• Updated the metrics window to draw both the unclamped and clamped legend rectangles for debugging. (implot3d.cpp)

Demo and usability improvements:

• Enhanced the legend demo with a slider to add dummy items, allowing users to test and visualize legend scrolling. (implot3d_demo.cpp) [1] [2]