Add workaround for gremlin'd wall collisions

src/p_test.cpp

@@ -15,6 +15,8 @@
 #include "p_sweep.hpp"
 
 #include "p_local.h"
+#include "g_demo.h"
+#include "r_main.h"
 
 consvar_t cv_showgremlins = CVAR_INIT ("showgremlins", "Off", 0, CV_OnOff, NULL);
 
@@ -62,13 +64,39 @@ line_t* P_SweepTestLines(fixed_t ax, fixed_t ay, fixed_t bx, fixed_t by, fixed_t
 		}
 	}
 
-	g_lines.clear();
-
 	if (!collision)
 	{
-		return nullptr;
+		line_t *line = nullptr;
+
+		if (!g_lines.empty() && !G_CompatLevel(0x000E))
+		{
+			// FIXME: This condition is a failsafe!
+			// SlopeAABBvsLine::vs_slope can sometimes report
+			// no collision despite P_CheckPosition saying otherwise.
+			// (Generally related to infinitesimal numbers or overflows.)
+			// When it reports no collision, that means no line and
+			// no normals to base the collision off of.
+			// Here we provide the last line checked and normals based on
+			// the line and the player's momentum angle.
+			// But the proper fix would be to make vs_slope work!!
+
+			line = g_lines.back();
+
+			angle_t lineangle = line->angle;
+			angle_t mobjangle = R_PointToAngle2(ax, ay, bx, by);
+			angle_t diff = lineangle - mobjangle;
+
+			lineangle += diff > ANGLE_180 ? -ANGLE_90 : ANGLE_90;
+
+			return_normal->x = FINECOSINE((lineangle >> ANGLETOFINESHIFT) & FINEMASK);
+			return_normal->y = FINESINE((lineangle >> ANGLETOFINESHIFT) & FINEMASK);
+		}
+
+		g_lines.clear();
+		return line;
 	}
 
+	g_lines.clear();
 	return_normal->x = Fixed {collision->normal.x};
 	return_normal->y = Fixed {collision->normal.y};