Add a new WebGLPointsLayer type using the shader builder utilities

This required adding a `a_index` attribute in the points layer renderer
to be able to make the precomputed shaders to work.

doc/errors/index.md

@@ -240,3 +240,7 @@ Support for the `OES_element_index_uint` WebGL extension is mandatory for WebGL
 ### 64
 
 Layer opacity must be a number.
+
+### 65
+
+A symbol literal representation must be defined on the style supplied to a `WebGLPointsLayer` instance.