[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/program.hpp

@@ -11,11 +11,13 @@ enum class ProgramDefines : bool {
     Overdraw = true,
 };
 
-template <class Shaders, class As, class Us>
-class Program : public gl::Program<As, Us> {
+template <class Shaders, class Primitive, class Attributes, class Uniforms>
+class Program : public gl::Program<Primitive, Attributes, Uniforms> {
 public:
+    using ParentType = gl::Program<Primitive, Attributes, Uniforms>;
+
     Program(gl::Context& context, ProgramDefines defines)
-        : gl::Program<As, Us>(context, Shaders::vertexSource, fragmentSource(defines))
+        : ParentType(context, Shaders::vertexSource, fragmentSource(defines))
         {}
 
     static std::string fragmentSource(ProgramDefines defines) {