render: Fix build with gcc 12

The xserver fails to compile with the latest gcc 12:

 render/picture.c: In function ‘CreateSolidPicture’:
 render/picture.c:874:26: error: array subscript ‘union _SourcePict[0]’ is partly outside array bounds of ‘unsigned char[16]’ [-Werror=array-bounds]
  874 |     pPicture->pSourcePict->type = SourcePictTypeSolidFill;
      |                          ^~
 render/picture.c:868:45: note: object of size 16 allocated by ‘malloc’
  868 |     pPicture->pSourcePict = (SourcePictPtr) malloc(sizeof(PictSolidFill));
      |                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 render/picture.c: In function ‘CreateLinearGradientPicture’:
 render/picture.c:906:26: error: array subscript ‘union _SourcePict[0]’ is partly outside array bounds of ‘unsigned char[32]’ [-Werror=array-bounds]
  906 |     pPicture->pSourcePict->linear.type = SourcePictTypeLinear;
      |                          ^~
 render/picture.c:899:45: note: object of size 32 allocated by ‘malloc’
  899 |     pPicture->pSourcePict = (SourcePictPtr) malloc(sizeof(PictLinearGradient));
      |                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 render/picture.c: In function ‘CreateConicalGradientPicture’:
 render/picture.c:989:26: error: array subscript ‘union _SourcePict[0]’ is partly outside array bounds of ‘unsigned char[32]’ [-Werror=array-bounds]
  989 |     pPicture->pSourcePict->conical.type = SourcePictTypeConical;
      |                          ^~
 render/picture.c:982:45: note: object of size 32 allocated by ‘malloc’
  982 |     pPicture->pSourcePict = (SourcePictPtr) malloc(sizeof(PictConicalGradient));
      |                                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 cc1: some warnings being treated as errors
 ninja: build stopped: subcommand failed.

This is because gcc 12 has become stricter and raises a warning now.

Fix the warning/error by allocating enough memory to store the union
struct.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Acked-by: Michel Dänzer <mdaenzer@redhat.com>
Closes: xorg/xserver#1256
(cherry picked from commit c6b0dcb8)
5 jobs for xserver-21.1-fix-build-with-gcc-12 in 4 minutes and 22 seconds (queued for 1 second)