mesa: raise max texture sizes to 16K

mesa: raise max texture sizes to 16K

This allows 16K x 16K 2D textures, for example, but we don't want to
allow that for 3D textures.  The new gl_constants::MaxTextureMBytes
field is used to prevent allocating too large of texture image.
This allows a 16K x 32 x 32 3D texture, for example, but prevents 16K^3.
Drivers can override this limit.  The default is currently 1GB.

Apps should use the proxy texture mechanism to determine the actual
max texture size.