Stacking blocks

Python 3, 89

def f(a):
 h=[]
 while a:x,w,*a=a;h[:x+w]=(h+[0]*x)[:x]+[max(h[x:x+w]+[0])+1]*w
 return h

Try it online.

The function takes and returns a list of integers.

def f(a):                       # input as list of integers
  h=[]                          # list of heights
  while a:                      # while there's input left
    x,w,*a=a;                   # pop first 2 integers as x and w

    h[:x+w]=                    # change the heights between 0 and x+w
      (h+[0]*x)[:x]+            # left of x -> unchanged but padded with zeros
      [max(h[x:x+w]+[0])+1]*w   # between x and x+w -> set to the previous max + 1

  return h                      # return the list of heights

Ruby, 88 87 bytes

f=->i{o=[]
(s,l,*i=i
r=s...s+l
o[r]=[([*o[r]]+[0]).max+1]*l
o.map! &:to_i)while i[0]
o}

Try it online.

Inspired by grc's answer, but in a different language and just slightly shorter.

Explanation:

f=->i                        # lambda with parameter i, expects array of ints
{
    o=[]                     # output
    (
        s,l,*i=i             # pop start and length
        r = s...s+l          # range is used twice, so shorten it to 1 char
        o[r] =
            [(
                    [*o[r]]  # o[r] returns nil if out of bounds, so splat it into another array
                    +[0]     # max doesn't like an empty array, so give it at least a 0
            ).max+1]*l       # repeat max+1 to fill length
        o.map! &:to_i        # replace nil values with 0
    ) while i[0]             # i[0] returns nil when i is empty, which is falsy
    o                        # return o
}

CJam, 34 30 bytes

Lq~2/{eeWf%e~_2$:).*:e>f*.e>}/

Input as a CJam-style array, output as a string of digits.

Run all test cases.

Here are two variants of another idea, but it's currently 2 bytes longer:

Lq~2/{_:\3$><0+:e>)aeez.*e_.e>}/
LQ~2/{_:\3$><0+:e>)aeez.+e~.e>}/