ES31: Implement EXT_geometry_shader framebuffer default layers on OpenGL This patch implements FRAMEBUFFER_DEFAULT_LAYERS_EXT as a legal framebuffer parameter required in OpenGL ES 3.1 extension EXT_geometry_shader on OpenGL back-ends. The query on FRAMEBUFFER_DEFAULT_LAYERS_EXT specifies the layer count used when the framebuffer has no attachments. BUG=angleproject:1941 TEST=dEQP-GLES31.functional.geometry_shading.query.framebuffer_default_layers Change-Id: I888465dfe23da53541ec2fedb8616027df532466 Reviewed-on: https://chromium-review.googlesource.com/1063560 Reviewed-by: Jiajia Qin <jiajia.qin@intel.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>