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implement HSV color mode and add colorMode() function

	it is now possible to specify colors in HSV format by first calling `colorMode(HSV)`
	The two constants RGB and HSV are used to switch between color modes.

--- a/a.h

+++ b/a.h

@@ -15,7 +15,7 @@

 void	initstate(lua_State*);

 void	registerapi(lua_State*);

-Image*	color(int, int, int);

+Image*	color(int, int, int, int);

 extern int		drawing;

 extern Image	*canvas;

--- a/api.c

+++ b/api.c

@@ -1,8 +1,15 @@

 #include "a.h"

+enum

+{

+	Crgb,

+	Chsv,

+};

+

 Image	*canvas;

 int		width;

 int		height;

+int		colormode;

 int		nostroke;

 int		strokecap;

 Image	*stroke;

@@ -20,6 +27,7 @@

 	height = 500;

 	r = Rect(0, 0, width, height);

 	canvas = allocimage(display, r, screen->chan, 0, DWhite);

+	colormode = Crgb;

 	nostroke = 0;

 	strokecap = Endsquare;

 	stroke = display->black;

@@ -63,22 +71,35 @@

 	c = lua_gettop(L);

 	if(c == 1){

-		r = luaL_checkinteger(L, 1);

-		g = r;

-		b = r;

+		g = luaL_checkinteger(L, 1);

+		i = color(g, g, g, 0);

 	}else if(c == 3){

 		r = luaL_checkinteger(L, 1);

 		g = luaL_checkinteger(L, 2);

 		b = luaL_checkinteger(L, 3);

+		i = color(r, g, b, colormode == Chsv);

 	}else{

 		fprint(2, "invalid color request\n");

 		return nil;

 	}

-	i = color(r, g, b);

 	return i;

 }

 int

+ccolormode(lua_State *L)

+{

+	int n;

+

+	n = luaL_checkinteger(L, 1);

+	if(n != Crgb && n != Chsv){

+		fprint(2, "error: invalid color mode\n");

+		return LUA_ERRRUN;

+	}

+	colormode = n;

+	return LUA_OK;

+}

+

+int

 cbackground(lua_State *L)

 {

 	Image *i;

@@ -354,10 +375,13 @@

 void

 registerapi(lua_State *L)

 {

+	lset(L, "RGB", Crgb);

+	lset(L, "HSV", Chsv);

 	lset(L, "SQUARE", Endsquare);

 	lset(L, "ROUND", Enddisc);

 	registerfunc(L, "size", csize);

+	registerfunc(L, "colorMode", ccolormode);

 	registerfunc(L, "background", cbackground);

 	registerfunc(L, "noStroke", cnostroke);

 	registerfunc(L, "strokeCap", cstrokecap);

--- a/color.c

+++ b/color.c

@@ -22,13 +22,79 @@

 	return i;

 }

+void

+hsvtorgb(int h, int s, int v, int *r, int *g, int *b)

+{

+	double dh, ds, dv, hh, p, q, t, ff;

+	long i;

+

+	dh = h;

+	ds = s/100.0;

+	dv = v/100.0;

+	if(ds <= 0.0){

+		*r = v;

+		*g = v;

+		*b = v;

+	}

+	hh = dh;

+	if(hh >= 360.0)

+		hh = 0.0;

+	hh /= 60.0;

+	i = (long)hh;

+	ff = hh - i;

+	p = dv * (1.0 - ds);

+	q = dv * (1.0 - (ds * ff));

+	t = dv * (1.0 - (ds * (1.0 - ff)));

+

+	switch(i) {

+		case 0:

+			*r = 255 * dv;

+			*g = 255 * t;

+			*b = 255 * p;

+			break;

+		case 1:

+			*r = 255 * q;

+			*g = 255 * dv;

+			*b = 255 * p;

+			break;

+		case 2:

+			*r = 255 * p;

+			*g = 255 * dv;

+			*b = 255 * t;

+			break;

+		case 3:

+			*r = 255 * p;

+			*g = 255 * q;

+			*b = 255 * dv;

+			break;

+		case 4:

+			*r = 255 * t;

+			*g = 255 * p;

+			*b = 255 * dv;

+			break;

+		case 5:

+		default:

+			*r = 255 * dv;

+			*g = 255 * p;

+			*b = 255 * q;

+			break;

+	}

+}

+

 Image*

-color(int r, int g, int b)

+color(int x, int y, int z, int hsv)

 {

 	ulong n;

 	Image *c;

-	int i;

+	int i, r, g, b;

+	if(hsv)

+		hsvtorgb(x, y, z, &r, &g, &b);

+	else{

+		r = x;

+		g = y;

+		b = z;

+	}

 	c = nil;

 	n = (r << 24) | (g << 16) | (b << 8) | 0xFF;

 	for(i = 0; i < nelem(cache); i++){