gallium: Define PIPE_TEXTURE_USAGE_DYNAMIC.

Knowing how the client intends to use the texture will give the
driver the opportunity to optimize for such cases.

src/gallium/include/pipe/p_defines.h

@@ -171,6 +171,7 @@ enum pipe_texture_target {
 #define PIPE_TEXTURE_USAGE_PRIMARY         0x4 /* ie a frontbuffer */
 #define PIPE_TEXTURE_USAGE_DEPTH_STENCIL   0x8
 #define PIPE_TEXTURE_USAGE_SAMPLER         0x10
+#define PIPE_TEXTURE_USAGE_DYNAMIC         0x20
 /** Pipe driver custom usage flags should be greater or equal to this value */
 #define PIPE_TEXTURE_USAGE_CUSTOM          (1 << 16)