some bugs in `subplot begin -F` with modifiers

[Dengda98]

1. +g behaves strange with +w , but single modifer works fine.

gmt begin panels png
  gmt subplot begin 2x2 -Ff6c+w1p,-+gyellow -R0/10/0/10
    gmt basemap -c
  gmt subplot end
gmt end show

2. +f should be at the end of subplot -F, otherwise the new layout cannot be processed correctly.

gmt begin panels png
  gmt subplot begin 2x2 -Ff6c+f3,1/1,2+w1p -R0/10/0/10
    gmt basemap -c
  gmt subplot end
gmt end show
# subplot [WARNING]: 2+w1 not a valid number and may not be decoded properly.

3. +g doesn't have the same effect with and without +c

gmt begin panels png
  gmt subplot begin 2x2 -Ff6c+gyellow+f3,1/1,2 -R0/10/0/10
    gmt basemap -c
  gmt subplot end
gmt end show

gmt begin panels png
  gmt subplot begin 2x2 -Ff6c+gyellow+c1c+f3,1/1,2 -R0/10/0/10
    gmt basemap -c
  gmt subplot end
gmt end show