gles2: Fixed the compilation problem and some bugs.

Previous patch doesn't set the offset to zero for GLESv2
path. Now fix it.

This patch also fix a minor problem in pixmap uploading
preparation. If the revert is not REVERT_NORMAL, then we
don't need to prepare a fbo for it. As current mesa i965
gles2 driver doesn't support to set a A8 texture as a fbo
target, we must fix this problem. As some A1/A8 picture
need to be uploaded, this is the only place a A8 texture
may be attached to a fbo.

This patch also enable the shader gradient for GLESv2.
The reason we disable it before is that some glsl linker
doesn't support link different objects which have cross
reference. Now we don't have that problem.

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>

glamor/glamor_glext.h

@@ -57,5 +57,8 @@
 #define GL_FRAMEBUFFER_INCOMPLETE_MULTISAMPLE 0x8D56
 
 #define GL_PACK_INVERT_MESA               0x8758
+#define GL_MAP_UNSYNCHRONIZED_BIT         0x0020
+#define GL_MAP_READ_BIT                   0x0001
+#define GL_MAP_WRITE_BIT                  0x0002
 
 #endif