x11-wm/wayfire-plugins-extra: convert LIBRSVG2 option to subpackage

[jbeich]

Summary

windecor plugin from librsvg2 subpackage can be used without the rest of wayfire-plugins-extra

Test Plan

$ poudriere testport -j 132i386 x11-wm/wayfire-plugins-extra
[...]
===========================================================================
=>> Checking shared library dependencies
 0x00000001 NEEDED               Shared library: [libc++.so.1]
 0x00000001 NEEDED               Shared library: [libc.so.7]
 0x00000001 NEEDED               Shared library: [libcairo.so.2]
 0x00000001 NEEDED               Shared library: [libcxxrt.so.1]
 0x00000001 NEEDED               Shared library: [libgcc_s.so.1]
 0x00000001 NEEDED               Shared library: [libgiomm-2.4.so.1]
 0x00000001 NEEDED               Shared library: [libglib-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libglibmm-2.4.so.1]
 0x00000001 NEEDED               Shared library: [libm.so.5]
 0x00000001 NEEDED               Shared library: [libsigc-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libwayland-server.so.0]
 0x00000001 NEEDED               Shared library: [libwf-config.so.1]
 0x00000001 NEEDED               Shared library: [libwlroots.so.11]

$ poudriere testport -j 132i386 x11-wm/wayfire-plugins-extra~librsvg2
[...]
===========================================================================
=>> Checking shared library dependencies
 0x00000001 NEEDED               Shared library: [libc++.so.1]
 0x00000001 NEEDED               Shared library: [libc.so.7]
 0x00000001 NEEDED               Shared library: [libcairo.so.2]
 0x00000001 NEEDED               Shared library: [libcxxrt.so.1]
 0x00000001 NEEDED               Shared library: [libgcc_s.so.1]
 0x00000001 NEEDED               Shared library: [libgio-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libgobject-2.0.so.0]
 0x00000001 NEEDED               Shared library: [libm.so.5]
 0x00000001 NEEDED               Shared library: [libpango-1.0.so.0]
 0x00000001 NEEDED               Shared library: [libpangocairo-1.0.so.0]
 0x00000001 NEEDED               Shared library: [librsvg-2.so.2]
 0x00000001 NEEDED               Shared library: [libwf-config.so.1]
 0x00000001 NEEDED               Shared library: [libwlroots.so.11]