Use overdraw inspector for wireframe mode

[brunoabinader]

Our native implementation for wireframe mode differs from js significantly. While native implementation modifies the fragment color before passing it to shader code, js uses a OVERDRAW_INSPECTOR define that forces fragment color to e.g. black for fills. While this means a couple extra shader programs to compute, this would contribute to clean up native's Painter code.

Ref: mapbox/mapbox-gl-shaders#1

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