This was reported upstream to Chromium by way of http://www.viva64.com/en/b/0113/ (Item N6)

http://cgit.freedesktop.org/mesa/mesa/tree/src/mesa/drivers/common/meta.c#n2934

Lines 2943 - 2948
      if (ctx->DrawBuffer->Visual.redBits <= 8)
         return GL_UNSIGNED_BYTE;
      else if (ctx->DrawBuffer->Visual.redBits <= 8)
         return GL_UNSIGNED_SHORT;
      else
         return GL_FLOAT;

The logic appears incorrect, in that GL_UNSIGNED_SHORT will never be returned - instead, it will return GL_FLOAT.

Cursory inspection of the Mesa sources indicates it supports 16bpp, so should this be written instead as:
      if (ctx->DrawBuffer->Visual.redBits <= 8)
         return GL_UNSIGNED_BYTE;
      else if (ctx->DrawBuffer->Visual.redBits <= 16)
         return GL_UNSIGNED_SHORT;
      else
         return GL_FLOAT;