Fix subtitle formatting

[mgoral]

Some formats generate incorrect formatting.

MicroDVD:

{100}{125}{y:i}{y:b}{y:u}This is formatted text

produces:

{gsp_i_}{gsp_b_}{gsp_u_}This is formatted text{_gsp_b}{_gsp_i}{_gsp_u}

(opening and closing tags are in different orders)

MPL2:

[40][50]/This is formatted text|and it's second line

produces:

{gsp_i_}This is formatted text{gsp_nl}and it's second line

(no closing tag - should be before first linesep (| === {gsp_nl}))