Additionally checks in DetectFormat for TGA files

[brianpopow]

• If all color map spec bytes are zero when color map type is zero
• Width and height are not zero

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

The heuristic for detecting a TGA file now additionally checks:

• If all color map spec bytes are zero when color map type is zero.
• Width and height are not zero.

[JimBobSquarePants]

Lovely!