cogl-clip-stack: Round the coords when clipping to a window rect

The size and position of the window rectangle for clipping in
try_pushing_rect_as_window_rect is calculated by projecting the
rectangle coordinates. Due to rounding errors, this can end up with
slightly off numbers like 34.999999. These were then being cast
directly to an integer so it could end up off by one.

This uses a new macro called COGL_UTIL_NEARBYINT which is a
replacement for the C99 nearbyint function.

cogl/cogl-clip-stack.c

@@ -37,6 +37,7 @@
 #include "cogl-internal.h"
 #include "cogl-framebuffer-private.h"
 #include "cogl-journal-private.h"
+#include "cogl-util.h"
 
 void _cogl_add_path_to_stencil_buffer (floatVec2    nodes_min,
                                        floatVec2    nodes_max,
@@ -483,7 +484,10 @@ try_pushing_rect_as_window_rect (float x_1,
     SWAP (y_1, y_2);
 #undef SWAP
 
-  cogl_clip_push_window_rectangle (x_1, y_1, x_2 - x_1, y_2 - y_1);
+  cogl_clip_push_window_rectangle (COGL_UTIL_NEARBYINT (x_1),
+                                   COGL_UTIL_NEARBYINT (y_1),
+                                   COGL_UTIL_NEARBYINT (x_2 - x_1),
+                                   COGL_UTIL_NEARBYINT (y_2 - y_1));
   return TRUE;
 }
 

cogl/cogl-util.h

@@ -52,4 +52,12 @@ cogl_util_float_signbit (float x)
 }
 #endif
 
+/* This is a replacement for the nearbyint function which always
+   rounds to the nearest integer. nearbyint is apparently a C99
+   function so it might not always be available but also it seems in
+   glibc it is defined as a function call so this macro could end up
+   faster anyway. We can't just add 0.5f because it will break for
+   negative numbers. */
+#define COGL_UTIL_NEARBYINT(x) ((int) ((x) < 0.0f ? (x) - 0.5f : (x) + 0.5f))
+
 #endif /* __COGL_UTIL_H */