gl: Fix implicit leaking of shader object

gl: Fix implicit leaking of shader object

According to OpenGL spec, a shader object will only be flagged
for deletion unless it has been detached; when a program object
is deleted, those shader objects attached to it will be detached
but not deleted unless they have already been flagged for deletion.

So we shall detach a shader object before it is deleted, and delete
it before the program object is deleted best.