add local versions of d3dx9 libraries

add ndi building into workflow

CMake: On non-Apple unix, use find_package for OpenGL

add ability to change font size and background opacity (fixes #66)

update stock textures

change gl default shader to use a texture coordinate (parity with the…

… d3d backend)

move skip option to one level higher; document

add support for texture that contains the contents of the previous frame

add option to select audio device

also update miniaudio to v0.10.4

Update README.md: add libasound2-dev as dependency on Linux (#136)

* Update README.md: add libasound2-dev as dependency on Linux

* Update README.md: add libasound2-dev in the Linux section comment as well