qt: add possibility to disable anti-aliasing in SDFAARoundedTexture.frag

I have not created a new file to prevent cluttering, as discussed. I intend to generate a new file once the build system starts supporting defines for shaders. For now, antialiasing is unconditionally used and the antialiasing property is overridden as readonly.

Based on !6468 (merged), only the last commit is relevant.

Although this adds the possibility, due to lacking build system support I do not manually create a new file that defines ANTIALIASING but rather use it unconditionally in both SDFAARoundedTexture.frag and SDFAARoundedTexture_cropsupport.frag and override the antialiasing property as readonly true.

When the build system supports defines for the shaders, we can start respecting antialiasing and revert the readonly attribute.

Request review @chub.