gfauto: add support for different framebuffer sizes

[amakin-siru]

If a test carries out loads of calculations, it's sometimes necessary to reduce the framebuffer size to prevent timeout occurring. Gfauto should have an option for this.

[ghost]

Hi would you please tell me more about this issue.

[prath06]

to make Optimal, I think reducing the framebuffer size will be better

[paulthomson]

I think we will need to add another field to the shader.json file e.g. $framebuffer: {width: 32, height: 32}. We need to update GraphicsFuzz (i.e. the Java code in the graphicsfuzz repo) so that it accepts the field (it could just ignore it, but it must not crash). And then we need to update gfauto to use the values when generating the AmberScript.

ghost and prath06: Apologies, but I don't think this is a good first issue; it is not well-documented, and we don't currently have the resources to mentor contributors outside Google Summer of Code.