gfauto: read the framebuffer size from shader.json (#1184)
gfauto: read the framebuffer size from shader.json (#1184)

This change updates gfauto to read the framebuffer size from shader.json
when generating AmberScript.
google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz,google/graphicsfuzz