Use uint64_t instead of double for interpolation.

More effort to fix the macOS issue.  This could potentially result
in slight problens with the rightmost pixel, but will protect
against any weird FP issues and -ffast-math concerns.

src/conio/scale.c

@@ -816,19 +816,19 @@ static void
 interpolate_width(uint32_t const* src, uint32_t* dst, const int width, const int height, const int newwidth)
 {
 	int x, y;
-	const double mult = (double)width / newwidth;
+	const uint64_t mult = (((uint64_t)width) << 16) / newwidth;
 	uint32_t *s = dst;
 	const int wm1 = width - 1;
 
 	int srow_start = 0;
 	int drow_start = 0;
 	for (y = 0; y < height; y++) {
-		double xpos = 0.0;
+		uint64_t xpos = 0;
 		dst = &s[drow_start];
 		for (x = 0; x < newwidth; x++) {
 			// First, calculate which two pixels this is between.
-			const int xposi = xpos;
-			const uint16_t weight = xpos * 65536;
+			const int xposi = xpos >> 16;
+			const uint16_t weight = xpos & 0xFFFF;
 			const int yposi = srow_start + xposi;
 			if (weight == 0) {
 				// Exact match!
@@ -865,8 +865,8 @@ static void
 interpolate_height(uint32_t const* src, uint32_t* dst, const int width, const int height, const int newheight)
 {
 	int x, y;
-	const double mult = (double)height / newheight;
-	double ypos = 0;
+	const uint64_t mult = (((uint64_t)height) << 16) / newheight;
+	uint64_t ypos = 0;
 	int last_yposi = 0;
 	int ywn = width;
 	static uint32_t *nline = NULL;
@@ -893,8 +893,8 @@ interpolate_height(uint32_t const* src, uint32_t* dst, const int width, const in
 	memcpy(tline, src, width * sizeof(*tline));
 	memcpy(nline, src + width, width * sizeof(*tline));
 	for (y = 0; y < newheight; y++) {
-		const int yposi = ypos;
-		const uint16_t weight = ((uint32_t)(ypos * 65536)) & 0xffff;
+		const int yposi = ypos >> 16;
+		const uint16_t weight = ypos & 0xffff;
 		if (yposi != last_yposi) {
 			ywn += width;
 			last_yposi = yposi;