Textures created through a render target are flipped along the X axis.
For the same reason, screenshots must be flipped back as well ('IVideoDriver::createScreenShot').
This commit implements the same flipping concept in two places:
1. Batch rendering of images (mostly used by fonts/text)
2. In-world rendering of such textures
`GL_LINES` isn't suitable, because it makes lines between pairs of 2 vertices,
not loops around 3 vertices.
Support for OpenGL ES isn't simple, as it has no `glPolygonMode`. And showing broken
wireframe (i.e. with `GL_LINES`) would cause confusion.
- Actually it's MSAA I think, or perhaps the terms are equivalent
- I've made it fit into the existing Irrlicht architecture, but that has resulted in code duplication compared to my original "hacky" approach
- OpenGL 3.2+ and OpenGL ES 3.1+ are supported
- EDT_OPENGL3 is not required, EDT_OPENGL works too
- Helpful tutorial: https://learnopengl.com/Advanced-OpenGL/Anti-Aliasing, section "Off-screen MSAA"
- This may be rough around the edges, but in general it works
