package/mesa3d: allow to enable OpenGL texture float support

package/mesa3d: allow to enable OpenGL texture float support

While testing on a PC target with an HD6310 graphic card, the
OpenGL support was in "compat profile" mode (i.e OpenGL 2.1).

Extended renderer info (GLX_MESA_query_renderer):
    Vendor: X.Org (0x1002)
    Device: AMD PALM (DRM 2.49.0 / 4.10.4) (0x9802)
    Version: 17.0.2
    Accelerated: yes
    Video memory: 256MB
    Unified memory: no
    Preferred profile: compat (0x2)
    Max core profile version: 0.0
    Max compat profile version: 2.1
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 2.0
OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD PALM (DRM 2.49.0 / 4.10.4)
OpenGL version string: 2.1 Mesa 17.0.2
OpenGL shading language version string: 1.30

OpenGL ES profile version string: OpenGL ES 2.0 Mesa 17.0.2
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 1.0.16

It turn out that texture float support was missing...

Extended renderer info (GLX_MESA_query_renderer):
    Vendor: X.Org (0x1002)
    Device: AMD PALM (DRM 2.49.0 / 4.10.4) (0x9802)
    Version: 17.0.2
    Accelerated: yes
    Video memory: 256MB
    Unified memory: no
    Preferred profile: core (0x1)
    Max core profile version: 3.3
    Max compat profile version: 3.0
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 3.0
OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD PALM (DRM 2.49.0 / 4.10.4)
OpenGL core profile version string: 3.3 (Core Profile) Mesa 17.0.2
OpenGL core profile shading language version string: 3.30
OpenGL core profile context flags: (none)
OpenGL core profile profile mask: core profile

OpenGL version string: 3.0 Mesa 17.0.2
OpenGL shading language version string: 1.30
OpenGL context flags: (none)

Add a new option to let the user disable this feature if
it doesn't comply to the SGI license.
Add docs/patents.txt to MESA3D_LICENSE_FILES

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
[Thomas:
 - rewrap Config.in help text
 - add note that not all drivers are affected by the texture float
   option.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>