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implement pushMatrix() and popMatrix()

	pushMatrix() saves the current transformation matrix on the stack, popMatrix() restore the previously
	saved matrix. This allow to control the scope of transformations.

--- a/api.c

+++ b/api.c

@@ -1,5 +1,13 @@

 #include "a.h"

+typedef struct Tmatrix Tmatrix;

+

+struct Tmatrix

+{

+	Point origin;

+	double angle;

+};

+

 enum

 {

 	Crgb,

@@ -21,11 +29,16 @@

 Point	origin;

 double	angle;

+

+Tmatrix tstack[255] = {0};

+int ntstack;

+

 void

 initstate(lua_State *L)

 {

 	Rectangle r;

+	ntstack = 0;

 	looping = 1;

 	framerate = 90;

 	width = 500;

@@ -429,6 +442,28 @@

 	return 1;

 }

+int

+cpushmatrix(lua_State *L)

+{

+	if(ntstack == nelem(tstack) - 1)

+		return luaL_error(L, "stack overflow");

+	tstack[ntstack].origin = origin;

+	tstack[ntstack].angle  = angle;

+	ntstack++;

+	return 0;

+}

+

+int

+cpopmatrix(lua_State *L)

+{

+	if(ntstack == 0)

+		return luaL_error(L, "stack underflow");

+	ntstack--;

+	origin = tstack[ntstack].origin;

+	angle  = tstack[ntstack].angle;

+	return 0;

+}

+

 void

 registerfunc(lua_State *L, const char *name, int(*f)(lua_State*))

 {

@@ -468,5 +503,7 @@

 	registerfunc(L, "transpose", ctranspose);

 	registerfunc(L, "rotate", crotate);

 	registerfunc(L, "radians", cradians);

+	registerfunc(L, "pushMatrix", cpushmatrix);

+	registerfunc(L, "popMatrix", cpopmatrix);

 }