No longer use gamma correction

Text looks more correct without the gamma correction. With gamma correction
text looks too bold in dark mode and too light in light mode. This
behavior is a sign that gamma correction is not the right thing to do.

The problem is seen with gamma at 1.5 used previously but is even worst
if a gamma value of 2.2, the theoretical value, is used.

lib/font_renderer/font_renderer.h

@@ -32,7 +32,7 @@ typedef struct {
   int left, top, right, bottom;
 } FR_Clip_Area;
 
-FR_Renderer *   FR_Renderer_New(unsigned int flags, float gamma);
+FR_Renderer *   FR_Renderer_New(unsigned int flags);
 void            FR_Renderer_Free(FR_Renderer *);
 int             FR_Load_Font(FR_Renderer *, const char *filename);
 FR_Bitmap*      FR_Bitmap_New(FR_Renderer *, int width, int height);