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>

src/compiler/glsl/link_uniform_blocks.cpp

@@ -391,7 +391,7 @@ void
 link_uniform_blocks(void *mem_ctx,
                     struct gl_context *ctx,
                     struct gl_shader_program *prog,
-                    struct gl_shader **shader_list,
+                    struct gl_linked_shader **shader_list,
                     unsigned num_shaders,
                     struct gl_uniform_block **ubo_blocks,
                     unsigned *num_ubo_blocks,