mesa: raise MAX_VARYING (number of shader varying vars) to 16

16 is the limit for now because of various 32-bit bitfields.

(cherry picked from master, commit 4e762395ef)

src/mesa/main/config.h

@@ -187,7 +187,7 @@
 #define MAX_PROGRAM_TEMPS 256
 #define MAX_PROGRAM_ADDRESS_REGS 2
 #define MAX_UNIFORMS 1024  /**< number of vec4 uniforms */
-#define MAX_VARYING 8      /**< number of float[4] vectors */
+#define MAX_VARYING 16     /**< number of float[4] vectors */
 #define MAX_SAMPLERS MAX_TEXTURE_IMAGE_UNITS
 #define MAX_PROGRAM_INPUTS 32
 #define MAX_PROGRAM_OUTPUTS 32