This fixes some issues with instanced markers rendering, and adds a few performance improvements.

Fix marker picking for instanced markers:

• Fix MarkerPickingFilter to set correct divisor for instanced rendering
• All marker tests now run with both 'points' and 'instanced' rendering methods

Instanced marker performance improvements:

• Use triangle strip topology (4 vertices instead of 6, fewer vert shader invocations)
• Add v_total_size varying to eliminate redundant fragment shader calculations
• Use shader templating for conditional gl_FragDepth to enable Early-Z optimization for non-spherical markers

For canvas size clamping:

• Add floor at 50% of min marker size to keep edges visible when zoomed out
• Add cap at 50% of marker radius to prevent edges from dominating small markers
• Fixes highlight rendering issues in napari when zooming