glamor: Change to use system memory for write only access.
If the pixmap is write-only, then use a pbo mapping will not get too much benefit. And even worse, when the software rendering is access this mapped data range, it's much slower than just using a system memory. From the glamor_prepare_access glamor_finish_access view, we have two options here: option 1: 1.0 create a pbo 1.1 copy texture to the pbo 1.2 map the pbo to va 1.3 access the va directly in software rendering. 1.4 bind the pbo as unpack buffer & draw it back to texture. option 2: 2.0 allocate a block memory in system memory space. 2.1 read the texture memory to the system memory. 2.2 access the system memory and do rendering. 2.3 draw the system memory back to texture. In general, 1.1 plush 1.2 is much faster than 2.1. And 1.3 is slower than 2.2. 1.4 is faster than 2.3. If the access mode is read only or read write, option 1 may be fater, but if the access mode is write only. Then most of the time option 1 is much faster. Signed-off-by: Zhigang Gong <email@example.com>