Please describe the problem:
The lock transparency functionality exposed in the layer dock UI is used to
limit the paining operations to the alpha-opaque areas of a layer. Using pain
tools (pencil and paintbrush) recently stopped behaving the way it used to. 

Steps to reproduce:
1. create a new empty layer
2. create a circle selection and fill with grey, select none.
3. pick a 1x1 brush, select black fg color, pencil tool and set the brush
opacity to 40%
4. try painting. everything works as should.
5. lock transparency of the layer (set lock alpha channel checkbox in the layer
dock)
6. try paining again. the painting should be limited to the contour of the
circle, but it should look the same as in step 4. 

Actual results:
You get a 4x4 fully opaque brush painting. this is especially evident
misbehavior with the paintbrush tool as it's otherwise anti-aliased but with
alpha locked, it isn't.

Expected results:
The paininting should look the same as without alpha locking, but limitied to
the layer's alpha-channel defined contour.

Does this happen every time?
yes

Other information:
this is cvs head.