Replace `(type *)0` appearances with type casts or nullptr

[no-lex]

This construction is used to either implicitly coerce following added pointers to the type of the pointer in the construction, or to initialize new pointers as null ones. C++'s nullptr is the appropriate way to address the latter concern, and the former one can be done with an explicit type cast of the same type onto the following symbol.

Current appearances:

src/engine/world/octaedit.cpp:1545:    prefabmesh::vertex *v = (prefabmesh::vertex *)0;
src/engine/world/octaedit.cpp:1555:    glDrawRangeElements_(GL_TRIANGLES, 0, p.numverts-1, p.numtris*3, GL_UNSIGNED_SHORT, (ushort *)0);
src/engine/world/octaedit.cpp:1564:    glDrawRangeElements_(GL_TRIANGLES, 0, p.numverts-1, p.numtris*3, GL_UNSIGNED_SHORT, (ushort *)0);
src/engine/render/renderva.cpp:15:    drawtris(numindices, (ushort *)0 + va->eoffset + offset, va->minvert, va->maxvert);
src/engine/render/renderva.cpp:20:    drawtris(va->sky, (ushort *)0 + va->skyoffset, va->minvert, va->maxvert);
src/engine/render/renderva.cpp:497:    gle::vertexpointer(sizeof(vec), (const vec *)0);
src/engine/render/renderva.cpp:523:    glDrawRangeElements_(GL_TRIANGLES, 0, 8-1, 3*2*6, GL_UNSIGNED_SHORT, (ushort *)0);
src/engine/render/renderva.cpp:1392:    vertex *vdata = (vertex *)0;
src/engine/render/renderva.cpp:1566:            drawtris(len, (ushort *)0 + curbatch->va->eoffset + curbatch->offset, curbatch->es.minvert, curbatch->es.maxvert);
src/engine/render/renderva.cpp:2214:    vertex *vdata = (vertex *)0;
src/engine/render/renderva.cpp:2770:        drawtris(3*d.tris, (ushort *)0 + d.offset, d.minvert, d.maxvert);
src/engine/render/rendergl.cpp:1705:    gle::vertexpointer(sizeof(vec2), (const vec2 *)0, GL_FLOAT, 2);
src/engine/render/renderlights.cpp:1498:        gle::colorpointer(sizeof(GLfloat), (const void *)0, GL_FLOAT, 1);
src/include/begin_code.h:167:#define NULL ((void *)0)
src/shared/glemu.cpp:94:        glDrawRangeElements_(GL_TRIANGLES, offset*4, (offset + count)*4-1, count*6, GL_UNSIGNED_SHORT, (ushort *)0 + offset*6);
src/shared/glemu.cpp:387:        buf = (uchar *)0 + vbooffset;