Painter: Re-enable diagonal draw_line().

This code still needs clipping, but the basic concept works. It was disabled
since before the kernel had floating point support.

LibM/math.cpp

@@ -91,4 +91,9 @@ double acos(double)
     ASSERT_NOT_REACHED();
 }
 
+double fabs(double value)
+{
+    return value < 0 ? -value : value;
+}
+
 }

Makefile.common

@@ -7,7 +7,7 @@ OPTIMIZATION_FLAGS = -Os
 MAKEFILE_PATH := $(abspath $(lastword $(MAKEFILE_LIST)))
 SERENITY_BASE_DIR := $(patsubst %/,%,$(dir $(MAKEFILE_PATH)))
 
-INCLUDE_FLAGS = -I$(SERENITY_BASE_DIR) -I. -I$(SERENITY_BASE_DIR)/LibC -I$(SERENITY_BASE_DIR)/Servers
+INCLUDE_FLAGS = -I$(SERENITY_BASE_DIR) -I. -I$(SERENITY_BASE_DIR)/LibC -I$(SERENITY_BASE_DIR)/Servers -I$(SERENITY_BASE_DIR)/LibM
 LDFLAGS = -L$(SERENITY_BASE_DIR)/LibC -L$(SERENITY_BASE_DIR)/LibCore -L$(SERENITY_BASE_DIR)/LibM -L$(SERENITY_BASE_DIR)/LibGUI
 CLANG_FLAGS = -Wconsumed -m32 -ffreestanding -march=i686
 #SUGGEST_FLAGS = -Wsuggest-final-types -Wsuggest-final-methods -Wsuggest-override #-Wsuggest-attribute=noreturn 

SharedGraphics/Painter.cpp

@@ -7,6 +7,7 @@
 #include <AK/StringBuilder.h>
 #include <unistd.h>
 #include <stdio.h>
+#include <math.h>
 
 Painter::Painter(GraphicsBitmap& bitmap)
     : m_target(bitmap)
@@ -511,9 +512,7 @@ void Painter::draw_line(const Point& p1, const Point& p2, Color color)
     }
 
     // FIXME: Implement clipping below.
-    ASSERT_NOT_REACHED();
 
-#if 0
     const double dx = point2.x() - point1.x();
     const double dy = point2.y() - point1.y();
     const double delta_error = fabs(dy / dx);
@@ -529,7 +528,6 @@ void Painter::draw_line(const Point& p1, const Point& p2, Color color)
             error -= 1.0;
         }
     }
-#endif
 }
 
 void Painter::draw_focus_rect(const Rect& rect)