viewport: consistently use floats for viewports

OpenGL < 4.0 only supports integer based viewports and internally we
have a mixture of code using floats and integers for viewports. This
patch switches all viewports throughout clutter and cogl to be
represented using floats considering that in the future we may want to
take advantage of floating point viewports with modern hardware/drivers.

cogl/cogl-framebuffer-private.h

@@ -46,10 +46,10 @@ struct _CoglFramebuffer
 
   CoglMatrixStack    *modelview_stack;
   CoglMatrixStack    *projection_stack;
-  int                 viewport_x;
-  int                 viewport_y;
-  int                 viewport_width;
-  int                 viewport_height;
+  float               viewport_x;
+  float               viewport_y;
+  float               viewport_width;
+  float               viewport_height;
 
   CoglClipState       clip_state;
 
@@ -173,25 +173,25 @@ _cogl_framebuffer_set_clip_stack (CoglFramebuffer *framebuffer,
 
 void
 _cogl_framebuffer_set_viewport (CoglFramebuffer *framebuffer,
-                                int x,
-                                int y,
-                                int width,
-                                int height);
-int
+                                float x,
+                                float y,
+                                float width,
+                                float height);
+float
 _cogl_framebuffer_get_viewport_x (CoglFramebuffer *framebuffer);
 
-int
+float
 _cogl_framebuffer_get_viewport_y (CoglFramebuffer *framebuffer);
 
-int
+float
 _cogl_framebuffer_get_viewport_width (CoglFramebuffer *framebuffer);
 
-int
+float
 _cogl_framebuffer_get_viewport_height (CoglFramebuffer *framebuffer);
 
 void
 _cogl_framebuffer_get_viewport4fv (CoglFramebuffer *framebuffer,
-                                   int *viewport);
+                                   float *viewport);
 
 CoglMatrixStack *
 _cogl_framebuffer_get_modelview_stack (CoglFramebuffer *framebuffer);