gl-renderer: add support for (a|x)bgr16161616f shm formats

These formats will be eventually be useful for color managed clients
using wl_shm that wish to submit buffers encoding high dynamic range
images.

While the minimum requirement for linearly filterable half float
textures is GL ES 2.0 + GL_OES_texture_half_float_linear, to keep
the code simple, this commit only enables the new formats when
the requirements for color management (notably including GL ES 3.0
and GL_EXT_color_buffer_half_float) are available.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>