There is a stack of matrices for each of the matrix modes.
In GL_MODELVIEW mode,
the stack depth is at least 32.
In the other two modes,
GL_PROJECTION and GL_TEXTURE ,
the depth is at least 2.
The current matrix in any mode is the matrix on the top of the stack
for that mode.
glPushMatrix pushes the current matrix stack down by one,
duplicating the current matrix.
That is,
after a glPushMatrix call,
the matrix on top of the stack is identical to the one below it.
glPopMatrix pops the current matrix stack,
replacing the current matrix with the one below it on the stack.
Initially, each of the stacks contains one matrix, an identity matrix.
It is an error to push a full matrix stack,
or to pop a matrix stack that contains only a single matrix.
In either case, the error flag is set
and no other change is made to GL state.
GL.glPushClientAttrib()