Eevee: Implement Overscan option

This option make the internal render size larger than the output size in
order to minimize screenspace effects disapearing at the render edges.

The overscan size added around the render is the maximum dimension
multiplied by the overscan percentage.

source/blender/render/extern/include/RE_pipeline.h

@@ -335,6 +335,7 @@ struct RenderPass *RE_pass_find_by_type(volatile struct RenderLayer *rl, int pas
 #define RE_BAKE_AO					2
 
 void RE_GetCameraWindow(struct Render *re, struct Object *camera, int frame, float mat[4][4]);
+void RE_GetCameraWindowWithOverscan(struct Render *re, float mat[4][4], float overscan);
 void RE_GetCameraModelMatrix(struct Render *re, struct Object *camera, float r_mat[4][4]);
 struct Scene *RE_GetScene(struct Render *re);
 void RE_SetScene(struct Render *re, struct Scene *sce);