glsl
Preprocessor prefix that opts a shader into dFdx / dFdy. Defines our private WW_HAS_DERIVATIVES macro plus, where needed, emits the GLES extension directive. Shaders gate the derivative call site with #ifdef WW_HAS_DERIVATIVES rather than the reserved GL_OES_standard_derivatives (GLSL forbids user #define of GL_* names).
Default value is appropriate for desktop GL (JOGL) and any GLES platform whose driver supports OES_standard_derivatives (the historical Android / iOS path). JS overrides for WebGL2 because that compiler refuses the #extension directive ("extension is not supported") even though derivatives are core there.