[core] Use gl::Program to resolve some rough edges in the GL binding types

* Extract `ignore` util to separate header.
* `Segment` now tracks offset and length of indices, rather than primitives. This is more natural.
* Introduce `VertexVector` and `IndexVector` types. These types carry information about the intended draw mode (`Triangles`, `LineStrip`, etc.), and ensure that elements are always appended in a group size appropriate for that draw mode, for both indexed and unindexed rendering.
* `Program`, rather than `Drawable`, is now the unifying object for draw calls. `Program` is the best place to type check the draw call, because it is typed to carry information about the intended primitive, vertex type, attributes, and uniforms.
* Use the debug shaders for debug tile rendering, like gl-js.
* Fix the draw mode for background. It was drawing triangle strips with a triangles array. Surprised this didn’t cause issues. Now it’s type checked.

src/mbgl/programs/raster_program.hpp

@@ -25,6 +25,7 @@ MBGL_DEFINE_UNIFORM_VECTOR(float, 2, u_tl_parent);
 
 class RasterProgram : public Program<
     shaders::raster,
+    gl::Triangle,
     gl::Attributes<
         attributes::a_pos,
         attributes::a_texture_pos>,