Raw-rs: add post-processing steps (#1923)

* add convert_to_rgb step

* add code to generate gamma correction curve

* add gamma correction step

* fix clippy warnings and cargo fmt

* remove unnecessary dependencies

* Code review 1

* Code review 2

* fix the order of operations

* Code review 3

---------

Co-authored-by: Keavon Chambers <keavon@keavon.com>

libraries/raw-rs/src/postprocessing/convert_to_rgb.rs

@@ -0,0 +1,40 @@
+use crate::Image;
+
+const CHANNELS_IN_RGB: usize = 3;
+
+pub fn convert_to_rgb(mut image: Image<u16>) -> Image<u16> {
+	let Some(rgb_to_camera) = image.rgb_to_camera else { return image };
+
+	// Rarely this might be 4 instead of 3 if an obscure Bayer filter is used, such as RGBE or CYGM, instead of the typical RGGB.
+	// See: <https://github.com/GraphiteEditor/Graphite/pull/1923#discussion_r1725070342>.
+	let channels = image.channels as usize;
+	let mut data = Vec::with_capacity(CHANNELS_IN_RGB * image.width * image.height);
+	let mut histogram = [[0; 0x2000]; CHANNELS_IN_RGB];
+
+	for i in 0..(image.height * image.width) {
+		let start = i * channels;
+		let end = start + channels;
+		let input_pixel = &mut image.data[start..end];
+
+		let mut output_pixel = [0.; CHANNELS_IN_RGB];
+		for (channel, &value) in input_pixel.iter().enumerate() {
+			output_pixel[0] += rgb_to_camera[0][channel] * value as f64;
+			output_pixel[1] += rgb_to_camera[1][channel] * value as f64;
+			output_pixel[2] += rgb_to_camera[2][channel] * value as f64;
+		}
+
+		for (output_pixel_channel, histogram_channel) in output_pixel.iter().zip(histogram.iter_mut()) {
+			let final_sum = (*output_pixel_channel as u16).clamp(0, u16::MAX);
+
+			histogram_channel[final_sum as usize >> CHANNELS_IN_RGB] += 1;
+
+			data.push(final_sum);
+		}
+	}
+
+	image.data = data;
+	image.histogram = Some(histogram);
+	image.channels = CHANNELS_IN_RGB as u8;
+
+	image
+}