use GL_EXT_gpu_shader4 when picking

OpenGL 2.1 does not include gl_PrimitiveId by default so we
try to enable it with an extension.

Change-Id: I2295d2de6cd180524a588e7af87c7ca207c996f9
Merge topic 'enable_picking_on_apple' into master

ed1381e8 use GL_EXT_gpu_shader4 when picking