LibGUI: Optimize invalidate and set alpha channel

If the window does not have an alpha channel, we now set every pixel's
alpha to 0xFF. This is needed by the WindowServer when it does alpha
blending, there used to be some weird stuff happening on overlapping
windows.

Also when we are invalidating a region with width of the whole window,
we can do a single memcpy instead of a memcpy for each row separately.

userspace/libraries/LibGUI/Window.cpp

@@ -11,6 +11,8 @@
 #include <sys/un.h>
 #include <unistd.h>
 
+#include <emmintrin.h>
+
 namespace LibGUI
 {
 
@@ -137,13 +139,47 @@ namespace LibGUI
 		return {};
 	}
 
+	static void* copy_pixels_and_set_max_alpha(void* dst, const void* src, size_t bytes)
+	{
+		size_t pixels = bytes / sizeof(uint32_t);
+
+		      __m128i* dst128 = static_cast<      __m128i*>(dst);
+		const __m128i* src128 = static_cast<const __m128i*>(src);
+		const __m128i alpha_mask = _mm_set1_epi32(0xFF000000);
+		for (; pixels >= 4; pixels -= 4)
+			_mm_storeu_si128(dst128++, _mm_or_si128(_mm_loadu_si128(src128++), alpha_mask));
+
+		      uint32_t* dst32 = reinterpret_cast<      uint32_t*>(dst128);
+		const uint32_t* src32 = reinterpret_cast<const uint32_t*>(src128);
+		for (; pixels; pixels--)
+			*dst32++ = *src32++ | 0xFF000000;
+
+		return nullptr;
+	}
+
 	void Window::invalidate(int32_t x, int32_t y, uint32_t width, uint32_t height)
 	{
 		if (!m_texture.clamp_to_texture(x, y, width, height))
 			return;
 
-		for (uint32_t i = 0; i < height; i++)
-			memcpy(&m_framebuffer_smo[(y + i) * m_width + x], &m_texture.pixels()[(y + i) * m_width + x], width * sizeof(uint32_t));
+		const auto copy_func = m_attributes.alpha_channel ? memcpy : copy_pixels_and_set_max_alpha;
+
+		if (width == m_width)
+		{
+			copy_func(
+				&m_framebuffer_smo[y * m_width],
+				&m_texture.pixels()[y * m_width],
+				width * height * sizeof(uint32_t)
+			);
+		}
+		else for (uint32_t y_off = 0; y_off < height; y_off++)
+		{
+			copy_func(
+				&m_framebuffer_smo[(y + y_off) * m_width + x],
+				&m_texture.pixels()[(y + y_off) * m_width + x],
+				width * sizeof(uint32_t)
+			);
+		}
 
 		WindowPacket::WindowInvalidate packet;
 		packet.x = x;