Compositor: Unify Variable Size Bokeh Blur node

This patch adjusts the Variable Size Bokeh Blur node such that it
matches between CPU and GPU. The GPU implementation is mostly followed
for the reasons stated below.

The first difference is a bug in the CPU implementation, where the upper
limit of the blur window is not considered, but the lower limit is.

The second difference is due to CPU ignoring outside pixels instead of
clamping them to border, which is done until an option is added to the
node to control the boundary condition.

The third difference is due to CPU ignoring the bounding box input.

The fourth difference is that CPU doesn't allow zero maximum blur
radius, which is a valid option.

The fifth difference is that the threshold option, which is only used
for the Defocus node, was considered in a greater than manner, while it
should be greater than or equal. Since the default threshold of one
should allow a blur size of one.

The GPU implementation now considers the maximum size of its input,
following the CPU implementation.

@@ -395,0 +289,4 @@
+  const float max_dim = std::max(get_width(), get_height());
+  const float base_size = do_size_scale_ ? (max_dim / 100.0f) : 1.0f;
+  const float maximum_size = size_buffer->get_max_value();
+  const int search_radius = math::clamp(0, int(maximum_size * base_size), max_blur_);

@@ -153,0 +159,4 @@
+    const float maximum_size = maximum_float(context(), input_size.texture());
+
+    const float base_size = compute_blur_radius();
+    return math::clamp(0, int(maximum_size * base_size), get_max_size());