Support dirty rects and old opengl rendering without shaders for basic games for jtransc homebrew targets

[soywiz]

[soywiz]

Relates to https://github.com/korlibs/korge/issues/768

[soywiz]

Some targets would require an extra effort to support opengl with shaders, since they are specific per platform. Some homebrew targets already support plain old opengl without shaders, so we could support these targets if we provide a fallback without shaders for simple rect rendering.

For targets without even opengl, we could draw sprites directly and even apply shaders via software, but instead of render all the sprites on every frame, we could detect dirty rects to paint only the things that changed from the previous frame (similar to what adobe flash does)