Stage.drawDebug does not work in combination with GL3 due to ShapeRenderer shader not being compliant

Issue details

When trying to draw UI debug stuff with OpenGL3 the application crashes due to the shape renderer not using GLES30 compliant GLSL syntax.

Reproduction steps/code

The configuration on Lwjgl3ApplicationConfiguration:

setOpenGLEmulation(Lwjgl3ApplicationConfiguration.GLEmulation.GL30, 3, 3)

I then enabled the debug drawing mode using:

stage.isDebugAll = isChecked

Version of libGDX and/or relevant dependencies

1.12.1

Stacktrace

Exception in thread "main" com.badlogic.gdx.utils.GdxRuntimeException: Error compiling shader: Vertex shader
ERROR: 0:1: '' :  #version required and missing.
ERROR: 0:1: 'attribute' : syntax error: syntax error
Fragment shader:
ERROR: 0:1: '' :  #version required and missing.
ERROR: 0:4: 'varying' : syntax error: syntax error

	at com.badlogic.gdx.graphics.glutils.ImmediateModeRenderer20.createDefaultShader(ImmediateModeRenderer20.java:242)
	at com.badlogic.gdx.graphics.glutils.ImmediateModeRenderer20.<init>(ImmediateModeRenderer20.java:55)
	at com.badlogic.gdx.graphics.glutils.ShapeRenderer.<init>(ShapeRenderer.java:116)
	at com.badlogic.gdx.graphics.glutils.ShapeRenderer.<init>(ShapeRenderer.java:111)
	at com.badlogic.gdx.graphics.glutils.ShapeRenderer.<init>(ShapeRenderer.java:107)
	at com.badlogic.gdx.scenes.scene2d.Stage.drawDebug(Stage.java:136)
	at com.badlogic.gdx.scenes.scene2d.Stage.draw(Stage.java:131)

Potential solution

Like in other places it would be useful to somehow tell the Stage what ShapeRenderer to use. Right now the attribute Stage.debugShapes (of type ShapeRenderer) is private and has no non private accessors. To support lazy creation an overrideable makeDebugShapeRenderer() could be offered to override the default behavior on demand.

Reproduce with Lwjgl3ApplicationConfiguration using GL30 emulation and Stage.isDebugAll, then trace Stage.drawDebug through ShapeRenderer and ImmediateModeRenderer20.createDefaultShader. Compare the shader failure in the reported stack trace with the GL3 requirements and verify that debug drawing no longer crashes under this configuration.