Gawain: increase size of immediate mode buffer

New value of 4MB should handle our needs without taking up too many GPU
resources.

Old value of 1KB was for observing what happens when the buffer fills up
and we need to flush and start a new one.

source/blender/gpu/intern/gpu_immediate.c

@@ -258,8 +258,7 @@ typedef struct {
 } Immediate;
 
 // size of internal buffer -- make this adjustable?
-// #define IMM_BUFFER_SIZE (4 * 1024 * 1024)
-#define IMM_BUFFER_SIZE 1024
+#define IMM_BUFFER_SIZE (4 * 1024 * 1024)
 
 static PER_THREAD bool initialized = false;
 static PER_THREAD Immediate imm;