Move true color sky drawing to its own drawers and chamge r_stretchsky to false as the new drawers can fade to a solid color

src/r_compiler/fixedfunction/drawskycodegen.h

@@ -0,0 +1,39 @@
+
+#pragma once
+
+#include "drawercodegen.h"
+
+enum class DrawSkyVariant
+{
+	Single,
+	Double
+};
+
+class DrawSkyCodegen : public DrawerCodegen
+{
+public:
+	void Generate(DrawSkyVariant variant, bool fourColumns, SSAValue args, SSAValue thread_data);
+
+private:
+	void Loop(DrawSkyVariant variant, bool fourColumns);
+	SSAVec4i Sample(SSAInt frac, int index, DrawSkyVariant variant);
+	SSAVec4i FadeOut(SSAInt frac, SSAVec4i color);
+
+	SSAStack<SSAInt> stack_index, stack_frac[4];
+
+	SSAUBytePtr dest;
+	SSAUBytePtr source0[4];
+	SSAUBytePtr source1[4];
+	SSAInt pitch;
+	SSAInt count;
+	SSAInt dest_y;
+	SSAInt texturefrac[4];
+	SSAInt iscale[4];
+	SSAInt textureheight0;
+	SSAInt textureheight1;
+	SSAVec4i top_color;
+	SSAVec4i bottom_color;
+	SSAWorkerThread thread;
+
+	SSAInt fracstep[4];
+};