Support SRGB

[Manishearth]

(From #83)

The hololens currently requests DXGI_FORMAT_B8G8R8A8_UNORM_SRGB as its most preferred format (and internally converts to srgb if you don't provide it). We should support directly producing srgb textures.

[jdm]

Since the openxr textures need to be compatible with the textures produced by surfman, this may require us to use the EGL_KHR_gl_colorspace extension to create surfaces using EGL_GL_COLORSPACE_SRGB_KHR.