BUILD: Conditionally include shaders
MYST3: Introduce shader-based renderer
MYST3: Move OpenGLTexture into separate file
GRAPHICS: Update system_headers.h for regular OpenGL
MYST3: Separate renderers into separate classes
JANITORIAL: Remove g_ prefix from instance variables
MYST3: Support Text rendering [OpenGLS]
MYST3: Reset viewport in menus
Merge pull request #834 from Botje/myst-es2

MYST3: ES2 backend