mesa: add GL_HALF_FLOAT as supported type to readpixels

mesa: add GL_HALF_FLOAT as supported type to readpixels

EXT_color_buffer_float spec states:

  "An INVALID_OPERATION error is generated ... if the color buffer is
   a floating-point format and type is not FLOAT, HALF FLOAT, or
   UNSIGNED_INT_10F_11F_11F_REV."

This means that GL_HALF_FLOAT type should be supported when color
buffer has floating-point format.

Fixes Android CTS test android.view.cts.PixelCopyTest.

v2: remove comments of EXT_color_buffer_half_float as
    EXT_color_buffer_float can use type GL_HALF_FLOAT

Signed-off-by: Lin Johnson <johnson.lin@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>