Include legacy shaders for rendering collision circles

[mpulkki-mapbox]

Debug shaders are being modified in mapbox/mapbox-gl-js#9219 as part of the collision circle refactoring. Original versions of collision_circle.vertex.glsl and collision_circle.fragment.glsl should be preserved for compatibility until the feature has been brought to gl-native.

Note: this is required after the PR has been merged in gl-js.

[stale]

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