Incorrect signature for GetPixelMapxv from OES_fixed_point

There appears to be a copy/paste error in OES_fixed_point, which states that GetPixelMapxv has a size parameter:

https://github.com/KhronosGroup/OpenGL-Registry/blob/e5778b0a610ccb29df46b2233daed4c1723c08e7/extensions/OES/OES_fixed_point.txt#L128-L129

(Note also that there's a missing comma between `int size` and `T* values`.)

But further down, it says that GetPixelMapxv is just the "x" version of GetPixelMap as defined in OpenGL 1.3:

https://github.com/KhronosGroup/OpenGL-Registry/blob/e5778b0a610ccb29df46b2233daed4c1723c08e7/extensions/OES/OES_fixed_point.txt#L352-L354

GetPixelMap does not have a size parameter:

> `void GetPixelMap{ui us f}v( enum map, T data );`

This error has snuck into gl.xml:

```xml

void glGetPixelMapxv
GLenum map
GLint size
GLfixed *values

```

The extension specification and gl.xml should be updated to remove the size parameter.

Compare extensions/OES/OES_fixed_point.txt with the OpenGL 1.3 GetPixelMap definition, then inspect gl.xml for glGetPixelMapxv. Done means the extension signature and XML command agree and no size parameter remains.

Written by the indexing model from the issue text.