The GLSL version code has been taken from ResidualVM. The variable
'texture' is now a reserved keyword in GLSL 3.00, so it has been
renamed. This fixes compilation issues in AmigaOS4 (PR 1554).
This fixes tons of warnings with clang from a recent xcode version on
macOS (and possibly other systems) complaining that an instantiation
of _singleton is required but no definition is available.
GLES2 requires precision qualifiers to be set and allows use of precision
qualifiers. For GLES2 we define a default precision now. Since precision
qualifiers are not supported in the GLSL version we use for GL, we
introduce compatibility #defines.
