This change uses half-precision floating-point implementation formats
for all OpenGL half-precision floating-point formats, as well as the
R11F_G11F_B10F and RGB9_E5 special internal formats.

sw::FORMAT_X16B16G16R16F was implemented for the formats without alpha.

RGB9E5 conversion was optimized to not require powf(2.0, x), and 11-
and 10-bit floating-point formats were optimized to map directly to
16-bit half-precision floating-point.

Change-Id: Ic33f903d01f37394244aec9f53b0e67d1c978764
Reviewed-on: https://swiftshader-review.googlesource.com/15410Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>