add gl.LINES mode primitives in samples

[andreasplesch]

as well as other primitive modes (for gltf 1.0 and gltf 2.0). Currently, all primitives in all sample models use gl.TRIANGLES mode (4). Here is a list of the other modes:

#define GL_POINTS                         0x0000
#define GL_LINES                          0x0001
#define GL_LINE_LOOP                      0x0002
#define GL_LINE_STRIP                     0x0003
//#define GL_TRIANGLES                      0x0004
#define GL_TRIANGLE_STRIP                 0x0005
#define GL_TRIANGLE_FAN                   0x0006

I found one gltf 2.0 example model which uses gl.LINES (1):

https://github.com/mrdoob/three.js/tree/dev/examples/models/gltf/OutlinedBox

donated by @twittmann.

gl.LINES is useful for technical drawings, engineering applications.

[javagl]

There once has been a glTF 1.0 test model using GL_LINES in three.js - sorry, I have to dig out and look up the best link, but this is from an earlier version that was part of a PR: https://github.com/bdysvik/three.js/tree/7eef51a27406bdfb24c29b4b1ff5f3303711ad59/examples/models/gltf/snowflake

Maybe this example can be used as a basis for a glTF 2.0 example as well.