stb_image: Support optional args consistently.

My guideline for the rules is the PNG loader (which I consider
"canonical"). In the _load functions, x and y are required but
comp is optional; in the _info functions, all three are optional.

Fixes issue #411 (and other related, unreported issues).

stb_image.h

@@ -5875,9 +5875,11 @@ static stbi_uc *stbi__pic_load_core(stbi__context *s,int width,int height,int *c
 static void *stbi__pic_load(stbi__context *s,int *px,int *py,int *comp,int req_comp, stbi__result_info *ri)
 {
    stbi_uc *result;
-   int i, x,y;
+   int i, x,y, internal_comp;
    STBI_NOTUSED(ri);
 
+   if (!comp) comp = &internal_comp;
+
    for (i=0; i<92; ++i)
       stbi__get8(s);
 
@@ -6496,6 +6498,11 @@ static int stbi__hdr_info(stbi__context *s, int *x, int *y, int *comp)
    char buffer[STBI__HDR_BUFLEN];
    char *token;
    int valid = 0;
+   int dummy;
+
+   if (!x) x = &dummy;
+   if (!y) y = &dummy;
+   if (!comp) comp = &dummy;
 
    if (stbi__hdr_test(s) == 0) {
        stbi__rewind( s );
@@ -6542,9 +6549,9 @@ static int stbi__bmp_info(stbi__context *s, int *x, int *y, int *comp)
    stbi__rewind( s );
    if (p == NULL)
       return 0;
-   *x = s->img_x;
-   *y = s->img_y;
-   *comp = info.ma ? 4 : 3;
+   if (x) *x = s->img_x;
+   if (y) *y = s->img_y;
+   if (comp) *comp = info.ma ? 4 : 3;
    return 1;
 }
 #endif
@@ -6552,7 +6559,10 @@ static int stbi__bmp_info(stbi__context *s, int *x, int *y, int *comp)
 #ifndef STBI_NO_PSD
 static int stbi__psd_info(stbi__context *s, int *x, int *y, int *comp)
 {
-   int channelCount;
+   int channelCount, dummy;
+   if (!x) x = &dummy;
+   if (!y) y = &dummy;
+   if (!comp) comp = &dummy;
    if (stbi__get32be(s) != 0x38425053) {
        stbi__rewind( s );
        return 0;
@@ -6585,9 +6595,13 @@ static int stbi__psd_info(stbi__context *s, int *x, int *y, int *comp)
 #ifndef STBI_NO_PIC
 static int stbi__pic_info(stbi__context *s, int *x, int *y, int *comp)
 {
-   int act_comp=0,num_packets=0,chained;
+   int act_comp=0,num_packets=0,chained,dummy;
    stbi__pic_packet packets[10];
 
+   if (!x) x = &dummy;
+   if (!y) y = &dummy;
+   if (!comp) comp = &dummy;
+
    if (!stbi__pic_is4(s,"\x53\x80\xF6\x34")) {
       stbi__rewind(s);
       return 0;
@@ -6673,7 +6687,7 @@ static void *stbi__pnm_load(stbi__context *s, int *x, int *y, int *comp, int req
 
    *x = s->img_x;
    *y = s->img_y;
-   *comp = s->img_n;
+   if (comp) *comp = s->img_n;
 
    if (!stbi__mad3sizes_valid(s->img_n, s->img_x, s->img_y, 0))
       return stbi__errpuc("too large", "PNM too large");
@@ -6727,16 +6741,20 @@ static int      stbi__pnm_getinteger(stbi__context *s, char *c)
 
 static int      stbi__pnm_info(stbi__context *s, int *x, int *y, int *comp)
 {
-   int maxv;
+   int maxv, dummy;
    char c, p, t;
 
-   stbi__rewind( s );
+   if (!x) x = &dummy;
+   if (!y) y = &dummy;
+   if (!comp) comp = &dummy;
+
+   stbi__rewind(s);
 
    // Get identifier
    p = (char) stbi__get8(s);
    t = (char) stbi__get8(s);
    if (p != 'P' || (t != '5' && t != '6')) {
-       stbi__rewind( s );
+       stbi__rewind(s);
        return 0;
    }