libgraphics: Fix blur algorithm

The blur algorithm had an error in the indexing for the vertical component of the blur and an error in the sliding window logic.

libraries/libgraphics/Framebuffer.cpp

@@ -126,14 +126,11 @@ void Framebuffer::copy_blitting(const Framebuffer& other, Rect other_area, const
 	}
 }
 
-inline int math_mod(int a, int b) {
-	return (a % b + b) % b;
-}
-
 void Framebuffer::blur(Gfx::Rect area, int radius) const {
 	int window_size = radius * 2 + 1;
+	area = area.overlapping_area(Rect {0, 0, width, height});
 
-	auto do_pass = [&]() { ;
+	auto do_pass = [&]() {
 		// First, apply blur horizontally.
 		for(int y = area.y; y < area.y + area.height; y++) {
 			int window[3] = {0, 0, 0};
@@ -155,11 +152,11 @@ void Framebuffer::blur(Gfx::Rect area, int radius) const {
 						(uint8_t) (window[1] / window_size),
 						(uint8_t) (window[2] / window_size),
 				};
-				auto window_add = data[(std::min(x + 5, width - 1)) + y * width];
+				auto window_add = data[(std::min(x + radius + 1, width - 1)) + y * width];
 				auto window_sub = window_preblur[preblur_index];
+				window_preblur[preblur_index] = window_add;
 				preblur_index++;
 				preblur_index %= window_size;
-				window_preblur[((preblur_index - 1) % window_size + window_size) % window_size] = window_add;
 				window[0] += (int) window_add.r - (int) window_sub.r;
 				window[1] += (int) window_add.g - (int) window_sub.g;
 				window[2] += (int) window_add.b - (int) window_sub.b;
@@ -174,7 +171,7 @@ void Framebuffer::blur(Gfx::Rect area, int radius) const {
 
 			// Populate window
 			for(int i = -radius; i <= radius; i++) {
-				auto color = data[(x + std::min(std::max(area.y + i, 0), height - 1)) * width];
+				auto color = data[x + (std::min(std::max(area.y + i, 0), height - 1)) * width];
 				window_preblur[i + radius] = color;
 				window[0] += color.r;
 				window[1] += color.g;
@@ -187,11 +184,11 @@ void Framebuffer::blur(Gfx::Rect area, int radius) const {
 						(uint8_t) (window[1] / window_size),
 						(uint8_t) (window[2] / window_size),
 				};
-				auto window_add = data[x + (std::min(y + 5, height - 1)) * width];
+				auto window_add = data[x + (std::min(y + radius + 1, height - 1)) * width];
 				auto window_sub = window_preblur[preblur_index];
+				window_preblur[preblur_index] = window_add;
 				preblur_index++;
 				preblur_index %= window_size;
-				window_preblur[((preblur_index - 1) % window_size + window_size) % window_size] = window_add;
 				window[0] += (int) window_add.r - (int) window_sub.r;
 				window[1] += (int) window_add.g - (int) window_sub.g;
 				window[2] += (int) window_add.b - (int) window_sub.b;