glsl/mesa: split gl_shader in two

glsl/mesa: split gl_shader in two

There are two distinctly different uses of this struct. The first
is to store GL shader objects. The second is to store information
about a shader stage thats been linked.

The two uses actually share few fields and there is clearly confusion
about their use. For example the linked shaders map one to one with
a program so can simply be destroyed along with the program. However
previously we were calling reference counting on the linked shaders.

We were also creating linked shaders with a name even though it
is always 0 and called the driver version of the _mesa_new_shader()
function unnecessarily for GL shader objects.

Acked-by: Iago Toral Quiroga <itoral@igalia.com>