Handle 16 bit PNG files in sRGB format

(cherry picked from commit e7e632cbedce3104e08b44525c6da78913fd8e53)

drivers/png/image_loader_png.cpp

@@ -51,7 +51,7 @@ Error ImageLoaderPNG::load_image(Ref<Image> p_image, FileAccess *f, bool p_force
 		f->close();
 	}
 	PoolVector<uint8_t>::Read reader = file_buffer.read();
-	return PNGDriverCommon::png_to_image(reader.ptr(), buffer_size, p_image);
+	return PNGDriverCommon::png_to_image(reader.ptr(), buffer_size, p_force_linear, p_image);
 }
 
 void ImageLoaderPNG::get_recognized_extensions(List<String> *p_extensions) const {
@@ -64,7 +64,8 @@ Ref<Image> ImageLoaderPNG::load_mem_png(const uint8_t *p_png, int p_size) {
 	Ref<Image> img;
 	img.instance();
 
-	Error err = PNGDriverCommon::png_to_image(p_png, p_size, img);
+	// the value of p_force_linear does not matter since it only applies to 16 bit
+	Error err = PNGDriverCommon::png_to_image(p_png, p_size, false, img);
 	ERR_FAIL_COND_V(err, Ref<Image>());
 
 	return img;

drivers/png/png_driver_common.cpp

@@ -58,8 +58,7 @@ static bool check_error(const png_image &image) {
 	return false;
 }
 
-Error png_to_image(const uint8_t *p_source, size_t p_size, Ref<Image> p_image) {
-
+Error png_to_image(const uint8_t *p_source, size_t p_size, bool p_force_linear, Ref<Image> p_image) {
 	png_image png_img;
 	zeromem(&png_img, sizeof(png_img));
 	png_img.version = PNG_IMAGE_VERSION;
@@ -100,6 +99,11 @@ Error png_to_image(const uint8_t *p_source, size_t p_size, Ref<Image> p_image) {
 			return ERR_UNAVAILABLE;
 	}
 
+	if (!p_force_linear) {
+		// assume 16 bit pngs without sRGB or gAMA chunks are in sRGB format
+		png_img.flags |= PNG_IMAGE_FLAG_16BIT_sRGB;
+	}
+
 	const png_uint_32 stride = PNG_IMAGE_ROW_STRIDE(png_img);
 	PoolVector<uint8_t> buffer;
 	Error err = buffer.resize(PNG_IMAGE_BUFFER_SIZE(png_img, stride));

drivers/png/png_driver_common.h

@@ -37,7 +37,7 @@
 namespace PNGDriverCommon {
 
 // Attempt to load png from buffer (p_source, p_size) into p_image
-Error png_to_image(const uint8_t *p_source, size_t p_size, Ref<Image> p_image);
+Error png_to_image(const uint8_t *p_source, size_t p_size, bool p_force_linear, Ref<Image> p_image);
 
 // Append p_image, as a png, to p_buffer.
 // Contents of p_buffer is unspecified if error returned.