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Updated Markers visual: instanced rendering and canvas size limits (#2701)

# Summary
This PR adds an optional `method` parameter to the `Markers` visual that
enables instanced rendering of markers using quads instead of
`GL_POINTS`. This works around platform-specific point size limitations
while maintaining full backward compatibility. Many OpenGL
implementations (macOS in particular) limit `GL_POINTS` to a maximum
size (often 64px or less), which prevents rendering large markers.
Applications like napari need to render markers at arbitrary sizes, and
exhibit inconsistent behavior across platforms. Instanced rendering
using quads allows for arbitrarily large markers where instanced
rendering is supported (most modern desktop hardware and operating
systems).

This is intended to fix #2078 

*Note: `method='instanced'` requires `vispy.use(gl="gl+")`, I'm not sure
how much of a limitation this is.*

## Changes
- Implemented instanced rendering using quads with per-instance
attributes
- Added `method` parameter to `Markers.__init__()` with options
`'points'` (default) or `'instanced'`
- Uses templates to share the bulk of the shader code between both
methods
- Both methods support all existing features: symbols, colors, edges,
scaling, spherical lighting
- Refactored `set_data()` to reduce complexity from 21 to 3
- Added an example to demonstrate the behavior changes, and validate the
appearance/performance of the new method

### New feature: Canvas size clamping
- Added `canvas_size_limits` property to clamp marker sizes in screen
pixels
- Eliminates need for downstream shader monkey-patching in napari
- Optional feature that works with both rendering methods, allowing
controlled consistent behavior across platforms (mimic the "aliasing"
effect on macOS with `GL_POINTS` in a predictable way)

## Testing & Backward Compatibility
The change is intended to be fully backward compatible, with the default
behavior unchanged. Marker tests are now parametrized... (continued)

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