Add obs_get_default_rect_effect

This provides a default effect for users of GL_TEXTURE_RECTANGLE/textures
that return true for gs_texture_is_rect
libobs-d3d11: Don't depend on specific D3DCompiler

I do not want the D3D11 library to depend on a specific compiler
version.  This way, I do not have to distribute D3D Compiler libraries
with the program (proprietary binary blobs).  Any particular version
works because the API for the D3DCompiler function appears to be the
same; the only things that change are other features and additions
mostly (at least as far as I can tell).  Using any version available on
the system should be more than sufficient rather than depending on some
specific D3D compiler version.

If the user doesn't have it, a download of the latest D3D distributables
should be fine, though it should work with the ones that come with
windows 7+ as well.
mac-capture: Rename display_capture::draw_effect to …::effect
mac-capture: Use obs_get_default_rect_effect
libobs/util: Write 0 character when converting empty strings

Fixes log_windows_version logging garbage for build when running with
debug heap enabled (assuming osvi.szCSDVersion is an empty string)