This adds support for custom shaders for polys, and properly handles modulate in the case of large FVF and modulate FVF.
It also fixes poly vertex colors not being sent to OpenGL.
Antialiased polys work by drawing a smoothed line around the poly after the main drawing. Batching draws polys as a series of triangles with no concept of 'edge', and when 2 polys are joined it becomes impractical to back calculate the edges from the triangles.
For this reason batching is disabled for antialiased polys in this PR.
As a result of the GLES specifications being vague about best practice for how buffers should be used dynamically, different GPUs / platforms appear to have different preferences.
Mac in particular seems to have a number of problems in this area, and none of the rendering team uses Macs. So far we have relied on guesswork to choose the best usage, but in an attempt to pin this down, this PR begins to introduce manual selection of options for users to test their configurations.
Lines are batched using the simplest fvf 'BatchVertex', however when used in an item with a custom shader material, it may attempt to translate to large_fvf without the required extra channels. To prevent this a special case in flushing is made to deal with lines.
In small batches using hardware transform, vertices would be drawn in incorrect positions due to the item transform being applied twice - once in the transform uniform, and once from the transform passed as a vertex attribute.
This PR alters the shader to ignore uniform transforms when using large FVF.
Batching is mostly separated into a common template which can be used with multiple backends (GLES2 and GLES3 here). Only necessary specifics are in the backend files.
Batching is extended to cover more primitives.
