Implement GL_OES_vertex_half_float.

Implement GL_OES_vertex_half_float.

This enables using half-float vertex data for OpenGL ES 2.0.
Note that GL_HALF_FLOAT_OES does not have the same value as
GL_HALF_FLOAT.

Change-Id: I0a7a55b7904fe797cdbb86627c8d0a1cc07d2d0a
Reviewed-on: https://swiftshader-review.googlesource.com/1500
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>