Use WEBGL_provoking_vertex and flat shading for attributes

[tuner]

Flat shading, which prevents unnecessary linear interpolation, is a performance disaster on Arm macs (see #119). However, there's a new extension that might solve the problem and allow for using flat shading: WEBGL_provoking_vertex

For instance, "point" mark's fragment shader has the following varyings, which are linearly interpolated between the vertices, even though they are same for all the vertices:

in float vRadius;
in float vRadiusWithPadding;
in lowp vec4 vFillColor;
in lowp vec4 vStrokeColor;
in lowp float vShape;
in lowp float vHalfStrokeWidth;
in mat2 vRotationMatrix;

Using flat shading would likely improve the performance a bit. Also, boolean and integer varyings can only be used with flat shading.