This is done by checking the number of bitplanes from the image buffer.
We assume that for float buffer to use the same bitplanes as it was a
byte buffer.
Then, the data of the image buffer is packed at the start of the `rect` or
`float_rect` before upload.
**Statistics - einar.v004.blend **
Note that not all grayscale textures have been stored as BW images so the
amount of memory that can be reduced would be more.
Without patch
```
104 Textures - 3294.99 MB (3294.47 MB over 32x32), 37 RTs - 192.52 MB.
Avg. tex dimension: 2201.88x1253.51 (2283.53x2202.13 over 32x32)
464 Buffers - 25.01 MB total 1.24 MB IBs 23.50 MB VBs.
3512.52 MB - Grand total GPU buffer + texture load
```
Patch applied
```
104 Textures - 2917.66 MB (2917.14 MB over 32x32), 39 RTs - 215.45 MB.
Avg. tex dimension: 2221.38x1252.75 (2323.28x2253.47 over 32x32)
467 Buffers - 25.01 MB total 1.24 MB IBs 23.51 MB VBs.
3158.13 MB - Grand total GPU buffer + texture load.
```
Reviewed By: fclem
Differential Revision: https://developer.blender.org/D15484
37533cd6cb