[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/debug_program.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <mbgl/programs/program.hpp>
+#include <mbgl/programs/attributes.hpp>
+#include <mbgl/programs/uniforms.hpp>
+#include <mbgl/shader/debug.hpp>
+
+namespace mbgl {
+
+class DebugProgram : public Program<
+    shaders::debug,
+    gl::Line,
+    gl::Attributes<
+        attributes::a_pos>,
+    gl::Uniforms<
+        uniforms::u_matrix,
+        uniforms::u_color>>
+{
+public:
+    using Program::Program;
+};
+
+} // namespace mbgl