Calling glDeleteShader() should mark shaders as pending for deletion,
but shouldn't decrement the refcount every time.  Otherwise, repeated
glDeleteShader() is not safe.

This is particularly bad since glDeleteProgram() frees shaders: if you
first call glDeleteShader() on the shaders attached to the program (thus
decrementing the refcount), then called glDeleteProgram(), it would try
to free them again (decrementing the refcount another time), causing
a refcount > 0 assertion to fail.

Similar to commit d950a778

.

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>