Previously, we initialized global variables that held pointers to the extension functions. While this seemed to work, the spec doesn't guarantee that the function pointers are identical for different OpenGL contexts. Therefore, we are now making them a member variable of the Context object.
This is safer now -- it can be an implementation detail of Segment/Context. And the typing of SegmentVector now ensures that the attributes and program match.
Two reasons to prefer explicit sequential location assignment, rather than relying on the GLSL linker to assign locations:
* For data-driven properties, we want to have the option to use glDisableVertexAttribArray plus glVertexAttrib*. In order to use glDisableVertexAttribArray, we must avoid using attribute location 0, which cannot be disabled.
* We want to use the same VAO in cases where, say, a fill layer might be rendered with FillProgram at first, and then FillPatternProgram later. VAOs do not store the program binding, only the attribute bindings, so as long as the two programs have the same attributes and attribute locations, only a single VAO (per segment) is needed.
* 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.
