The GL_OES_shader_image_atomic is now supported by mainline Mesa with all of the drivers that support the GL_ARB_shader_image_load_store extension. The GL_OES_shader_image_atomic extension is about providing atomic read-modify-write extensions on a single level of a texture object from any shader stage. More details about the extension here.
Within the Mesa 11.3-devel Git master code this morning, Francisco Jerez at Intel's Open-Source Technology Center landed the GLES 3.2 extension. There still are a number of OpenGL ES 3.2 extensions left to be completed by the various Mesa drivers. In fact, aside from the Intel i965 driver, there are a number of extensions of GLES 3.1 left to be implemented still for Radeon and Nouveau. Intel has taken more focus in OpenGL ES 3.x compliance rather than tackling the specific OpenGL 4.x work items for desktop compliance.