In gimp_drawable_gradient(), pass the undo description ("Gradient")
to gimp_gegl_apply_operation(), so that its displayed as the
progress text while rendering the gradient, even when applying a
shaped gradient, in which case we end the progress after
calculating the distance map, causing the progress text be NULL
during rendering unless explicitly set.
(cherry picked from commit deee2f14f5)
Symmetric conical gradients only span half a revolution (unlike
assymetric ones, which span an entire revolution), and therefore
require only half the cache size.
Add gimp_drawable_gradient_adjust_coords(), which adjusts the
gradient segment coords according to the gradient type, so that, in
cases where the gradient span is unrelated to the segment length,
the gradient cache (in GimpOperationGradient) is big enough not to
produce banding. Use the new function in gimp_drawable_gradient()
and in the gradient tool, instead of duplicating the logic.
Move the shapreburst coordinate-adjustment logic to the new
function, and add appropriate logic for conical gradients.
Remove the code that avoids using the gradient cache for conical
gradients from GimpOperationGradient.
In the various types of fill operations, and in fade operations,
use the paint composite-mode of the current paint mode, which is
the composite mode we use during painting, instead of AUTO, which
results in the default mode we use for layer compositing. This
effectively means that filling using any non-legacy, non-
subtractive mode can paint over transparent areas, rather than
being limited to nontransparent areas.
