[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/programs.hpp

@@ -5,6 +5,7 @@
 #include <mbgl/programs/line_program.hpp>
 #include <mbgl/programs/raster_program.hpp>
 #include <mbgl/programs/symbol_program.hpp>
+#include <mbgl/programs/debug_program.hpp>
 #include <mbgl/programs/collision_box_program.hpp>
 
 namespace mbgl {
@@ -24,6 +25,7 @@ public:
           symbolIcon(context, defines),
           symbolIconSDF(context, defines),
           symbolGlyph(context, defines),
+          debug(context, ProgramDefines::None),
           collisionBox(context, ProgramDefines::None) {
     }
 
@@ -40,6 +42,7 @@ public:
     SymbolSDFProgram symbolIconSDF;
     SymbolSDFProgram symbolGlyph;
 
+    DebugProgram debug;
     CollisionBoxProgram collisionBox;
 };