LibSoftGPU: Move alpha blend factor setup out of triangle rasterization

Since the alpha blend configuration should not change between most calls
of draw_primitives it makes no sense to reinitialize the blend factors
for every rasterized triangle.
The alpha blend factors are now set up whenever the device config
changes. The blend factors are stored in struct AlphaBlendFactors.

Userland/Libraries/LibSoftGPU/AlphaBlendFactors.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2021, Stephan Unverwerth <s.unverwerth@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGfx/Vector4.h>
+
+namespace SoftGPU {
+
+struct AlphaBlendFactors final {
+    FloatVector4 src_constant {};
+    float src_factor_src_alpha = 0;
+    float src_factor_dst_alpha = 0;
+    float src_factor_src_color = 0;
+    float src_factor_dst_color = 0;
+
+    FloatVector4 dst_constant {};
+    float dst_factor_src_alpha = 0;
+    float dst_factor_dst_alpha = 0;
+    float dst_factor_src_color = 0;
+    float dst_factor_dst_color = 0;
+};
+
+}